Implement song format spec

[bohning]

For details see https://usdx.eu/format/.

[RumovZ]

Regarding attributes not yet supported by USDB, they could potentially be parsed from the comments.

[Baklap4]

In a recent PR #313 some of the tags have been added, very nice!
I do however miss the following to make it complete

1. #PROVIDEDBY, for usdb_syncer this will always be: https://usdb.animux.de

   #PROVIDEDBY helps to understand where a txt-file cames from. This is important cause there are many third-parties that host txt-files with different quality standards. This information should be normally set automatically from the provider. But you can set it as well manually with a song editor if you know where you got it from. Value should be a URL.

aside from that, can this issue be closed?