#186 Add support for rendering .srt subtitles during encoding

[hemisfear]

It would be nice to be able to render/overlay .srt subtitles directly onto the video output stream during the encoding process.

For example:

A foreign language film in an arbitrary video format comes with a .srt subtitle file containing english subtitles. While ffmpeg is extremely useful for converting the film to a different format, the transcoding of the film is useless if the subtitles cannot be integrated into the target video format.