Repository: youtube-dl
Issue: Allow using ffmpeg -vf ( video filters ) in --external-downloader-args
Labels: request

## Please follow the guide below

- You will be asked some questions and requested to provide some information, please read them **carefully** and answer honestly
- Put an `x` into all the boxes [ ] relevant to your *issue* (like that [x])
- Use *Preview* tab to see how your issue will actually look like

---

### Make sure you are using the *latest* version: run `youtube-dl --version` and ensure your version is *2017.02.10*. If it's not read [this FAQ entry](https://github.com/rg3/youtube-dl/blob/master/README.md#how-do-i-update-youtube-dl) and update. Issues with outdated version will be rejected.
- [x ] I've **verified** and **I assure** that I'm running youtube-dl **2017.02.10**

### Before submitting an *issue* make sure you have:
- [x ] At least skimmed through [README](https://github.com/rg3/youtube-dl/blob/master/README.md) and **most notably** [FAQ](https://github.com/rg3/youtube-dl#faq) and [BUGS](https://github.com/rg3/youtube-dl#bugs) sections
- [ x] [Searched](https://github.com/rg3/youtube-dl/search?type=Issues) the bugtracker for similar issues including closed ones

### What is the purpose of your *issue*?
- [ x] Bug report (encountered problems with youtube-dl)
- [ ] Site support request (request for adding support for a new site)
- [ x] Feature request (request for a new functionality)
- [ ] Question
- [ ] Other

---

### The following sections concretize particular purposed issues, you can erase any section (the contents between triple ---) not applicable to your *issue*

---

### If the purpose of this *issue* is a *bug report*, *site support request* or you are not completely sure provide the full verbose output:

```
Option vf (set video filters) cannot be applied to input url http://livestream-f.akamaihd.net/i/23093026_6897778_lsiswg499e87c8vkhxz_1@407805/index_2320_av-b.m3u8?sd=10&dw=100&rebase=on&hdntl=exp=1486831046~acl=%2fi%2f23093026_6897778_lsiswg499e87c8vkhxz_1%40407805%2f*~data=hdntl~hmac=480a5086e32f3ffea64b61645ab7db0738f7e7e1c8000aeccd91de259aa8f63a -- you are trying to apply an input option to an output file or vice versa. Move this option before the file it belongs to.
Error parsing options for input file http://livestream-f.akamaihd.net/i/23093026_6897778_lsiswg499e87c8vkhxz_1@407805/index_2320_av-b.m3u8?sd=10&dw=100&rebase=on&hdntl=exp=1486831046~acl=%2fi%2f23093026_6897778_lsiswg499e87c8vkhxz_1%40407805%2f*~data=hdntl~hmac=480a5086e32f3ffea64b61645ab7db0738f7e7e1c8000aeccd91de259aa8f63a.
Error opening input files: Invalid argument


ERROR: ffmpeg exited with code 1
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/local/bin/youtube-dl/__main__.py", line 19, in <module>
    youtube_dl.main()
  File "/usr/local/bin/youtube-dl/youtube_dl/__init__.py", line 451, in main
    _real_main(argv)
  File "/usr/local/bin/youtube-dl/youtube_dl/__init__.py", line 441, in _real_main
    retcode = ydl.download(all_urls)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 1791, in download
    url, force_generic_extractor=self.params.get('force_generic_extractor', False))
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 705, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 751, in process_ie_result
    return self.process_video_result(ie_result, download=download)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 1435, in process_video_result
    self.process_info(new_info)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 1699, in process_info
    success = dl(filename, info_dict)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 1641, in dl
    return fd.download(name, info)
  File "/usr/local/bin/youtube-dl/youtube_dl/downloader/common.py", line 353, in download
    return self.real_download(filename, info_dict)
  File "/usr/local/bin/youtube-dl/youtube_dl/downloader/hls.py", line 77, in real_download
    return fd.real_download(filename, info_dict)
  File "/usr/local/bin/youtube-dl/youtube_dl/downloader/external.py", line 43, in real_download
    self.get_basename(), retval))
  File "/usr/local/bin/youtube-dl/youtube_dl/downloader/common.py", line 162, in report_error
    self.ydl.report_error(*args, **kargs)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 568, in report_error
    self.trouble(error_message, tb)
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 530, in trouble
    tb_data = traceback.format_list(traceback.extract_stack())

```

---

### Description of your *issue*, suggested solution and other information

I would like to add a timestamp to a live video stream that I'm downloading. The command I'm running is as follows: 

```
youtube-dl --hls-use-mpegts --external-downloader-args "-vf \"drawtext=fontfile=/Library/Fonts/Arial Black.ttf: \
text='%{localtime\:%c}': fontcolor=white@0.8: fontsize=24: box=1: boxcolor=black@0.5: \
boxborderw=5: x=(w-text_w)-10: y=10\"" "https://livestream.com/accounts/23093026/events/6897778/\
player?enableInfoAndActivity=true&autoPlay=true&mute=false&startswith=true"
```
I have tested the ffmpeg **-vf drawtext** arguments without youtube-dl and the timestamp is video filter is correctly added. example: 
```
ffmpeg -i test.mp4 -s 640x480 -r 30 \
-vf "drawtext=fontfile=/Library/Fonts/Arial Black.ttf: \
text='%{localtime\:%c}': fontcolor=white@0.8: fontsize=24: box=1: boxcolor=black@0.5: \
boxborderw=5: x=(w-text_w)-10: y=10" -vcodec libx264 -vb 2000k \
-preset ultrafast -f mp4 output.mp4
```

But the error above ( Option vf (set video filters) cannot be applied to input url ... ) shows up when i run the youtube-dl command above. Please can you tell me if you know of a way I can get a time stamp appended to my livestream download? Thank you very much.