Join the save_dir to the file_path while using ffmpeg

So I'm running this command locally:

`farleyknight@farleyknight-desktop:~/dev/ruby/rb2k/viddl-rb$ ruby bin/viddl-rb http://www.youtube.com/view_play_list?p=RD020bXB5zyF3TI --extract-audio --save-dir /home/farleyknight/Downloads/trap_youtube_mp3z/`

After a bit of processing, I get:

```
Download for Waka_Flocka___Cant_Do_Gold__Official_Music_Video_.mp4 successful.
Gathering information about the downloaded file.
Done gathering information about the downloaded file.
Error: ERROR: Couldn't find any audio:
"ffmpeg version 0.8.6-4:0.8.6-0ubuntu0.12.04.1, Copyright (c) 2000-2013 the Libav developers\n  built on Apr  2 2013 17:02:36 with gcc 4.6.3\n*** THIS PROGRAM IS DEPRECATED ***\nThis program is only provided for compatibility and will be removed in a future release. Please use avconv instead.\nWaka_Flocka___Cant_Do_Gold__Official_Music_Video_.mp4: No such file or directory\n"
```

So it seems that it's trying to find the file in the local directory, when it's actually saved in a separate directory. This pull request should fix that.