Log ffmpeg commands
Merge pull request #16 from tahajahangir/add-mp3-format

Add mp3 format to allow converting to mp3 files
Merge pull request #18 from tahajahangir/improve_logging

Log ffmpeg commands
Grab stream bitrate in MediaFormatInfo
Grab stream bitrate in MediaStreamInfo
Stream bitrate should be None when it's N/A on ffprobe output
Add tests for MediaStreamInfo.bitrate