Add scripting support for BPM ID3tag (to use in tagnavi.config)

[xplorr]

Please add support for reading and evaluating "BPM" (=tempo) ID3tag, just like "tracknum", "discnum" etc. (to use in tagnavi.config).
suggested name: "bpm"
This allows for custom menus (dynamic playlists) like:
"Tempo < 100" -> album ? bpm < "100" -> artist -> title = "fmt_titlenoartist"
This will create a custom menu with tracks with tempo (BPM) < 100