v3.1.0 — Add support for indexing bmson video.

bemuse-indexer will index the BGA for bmson charts:

• Caution: Due to the current structure of the Bemuse’s codebase, BGA is song-specific, not chart-specific. This means multiple BGAs in one song is not possible.

• The BGA will be enabled if there is only 1 BGA event in the song which should be a movie file (multiple BGAs in same chart is not possible).

• If multiple charts have a BGA definition, then the BGA is taken from one of the charts arbitrarily.

• This will result in these fields added to the song object:

  Field	Description
  video_file	The video file name as specified in the bga_header
  video_offset	The number of seconds into the song to start playing the video