Repository: youtube-dl
Issue: Allow rendering subtitles besides simply adding a subtitle stream (was: Feature request "-f bestsubtitle")
Labels: request,subtitles

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- Put an `x` into all the boxes [ ] relevant to your _issue_ (like that [x])
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---
### Make sure you are using the _latest_ version: run `youtube-dl --version` and ensure your version is _2016.05.30.2_. 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 Issues with outdated version will be rejected.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 **2016.05.30.2**
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- [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
- [ ] [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_?
- [ ] 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

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### The following sections concretize particular purposed issues, you can erase any section (the contents between triple ---) not applicable to your _issue_

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### If the purpose of this _issue_ is a _bug report_, _site support request_ or you are not completely sure provide the full verbose output as follows:

Add `-v` flag to **your command line** you run youtube-dl with, copy the **whole** output and insert it here. It should look similar to one below (replace it with **your** log inserted between triple ```):

```
$ youtube-dl -v <your command line>
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'-v', u'http://www.youtube.com/watch?v=BaW_jenozKcj']
[debug] Encodings: locale cp1251, fs mbcs, out cp866, pref cp1251
[debug] youtube-dl version 2016.05.30.2
[debug] Python version 2.7.11 - Windows-2003Server-5.2.3790-SP2
[debug] exe versions: ffmpeg N-75573-g1d0487f, ffprobe N-75573-g1d0487f, rtmpdump 2.4
[debug] Proxy map: {}
...
<end of log>
```

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### If the purpose of this _issue_ is a _site support request_ please provide all kinds of example URLs support for which should be included (replace following example URLs by **yours**):
- Single video: https://www.youtube.com/watch?v=BaW_jenozKc
- Single video: https://youtu.be/BaW_jenozKc
- Playlist: https://www.youtube.com/playlist?list=PL4lCao7KL_QFVb7Iudeipvc2BCavECqzc

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### Description of your _issue_, suggested solution and other information

there are already `-f` parameters `bestvideo` and `bestaudio` but i wonder why there is no `bestsub` or `bestsubtitle` to help burn subtitles into video files
For example:

``` bash
-f "bestvideo[ext=mp4]+bestaudio[ext=m4a]+bestsubtitle[ext=vtt]/best"
```

will burn both best audio and best subtitle into the mp4 file.
- `--write-auto-sub` option should be automatically used if `bestsubtitle` is invoked but no subtitle is available
- if `bestsubtitle` is invoked without `bestvideo` and `bestaudio`, download best subtitle only