Add ffmpeg concat postprocessor

Code's a bit sloppy, but it seems to work.
Implement initial concat support

It may not be pretty, but it friggin' works. Some initial testing with
The Daily Show, but I have no idea how it handles other sites.
Merge branch 'concat' of https://github.com/AGSPhoenix/youtube-dl into AGSPhoenix-concat

Conflicts:
	youtube_dl/postprocessor/ffmpeg.py