#419 Nashville Number System support

[David Mitchell]

I would like to see additional support for Nashville Number System chord notation (see attached one-page guide by Dan Wilt) in OpenSong. It appears that I can already use numbers for chords, but it would be excellent if we could use the transposition function to switch from standard chords to NNS chords. If you could offer both numbers and Roman numerals, it would satisfy request #411 as well!