Support indexing of additional filetypes

[ToxicFrog]

gonic version: 0.15.2

The indexer requires files to pass two checks to be indexed:

• The file extension must be one of the supportedAudioTypes
• taglib must successfully read ID3 or Vorbis tags from it.

The former means that files not in that set -- including all VGM and tracker formats, MIDI, and less popular PCM formats -- will be skipped entirely. The latter means that even if you try to include them by giving them fake extensions, it'll drop them from the scan when it tries to read metadata.

In principle, gonic should be able to play anything it can transcode, and it can transcode anything ffmpeg supports, which is a lot of formats even before you start getting into workarounds to let it use other commands for transcoding.

I think the most general way to support this would be:

• do not filter based on file extension (or if you do, make the list of supported extensions user-extensible)
• if taglib fails to read the tags, shell out to ffprobe -of json and parse the output

For extra credit, support a file extension to probe command mapping in configuration, but even without that the admin can replace ffprobe with a compatible wrapper for formats it doesn't support.