Accept more tags for track and disc number

- Accept 'DISCNUMBER' as a synonym for 'DISC'
 - Accept 'TRACK' as a synonum for 'TRACKNUMBER'

 This should help with #146

src/tangara/database/tag_parser.cpp

@@ -171,9 +171,11 @@ OggTagParser::OggTagParser() {
   nameToTag_["ALBUM"] = Tag::kAlbum;
   nameToTag_["ARTIST"] = Tag::kArtist;
   nameToTag_["ALBUMARTIST"] = Tag::kAlbumArtist;
+  nameToTag_["TRACK"] = Tag::kTrack;
   nameToTag_["TRACKNUMBER"] = Tag::kTrack;
   nameToTag_["GENRE"] = Tag::kGenres;
   nameToTag_["DISC"] = Tag::kDisc;
+  nameToTag_["DISCNUMBER"] = Tag::kDisc;
 }
 
 auto OggTagParser::ReadAndParseTags(std::string_view p)