Support Ukulele chord diagrams

[shannonwells]

User Story

So that a jam session leader can lead an ukulele jam session, jamsession can create song PDFs and songbooks with ukulele chords.

1. The chords for each song will be the same as for guitar, however the chord diagrams are obviously different. Uke chord support will be necessary.
2. Difficulty on ukulele is not always the same as with guitar, so difficulty would need to be adjustable depending on instrument
3. Need the ability to display chords for uke vs. guitar on individual songs
4. Need the ability to create a songbook for uke or guitar

Starting point

See https://github.com/spilth/chord_diagrams/blob/master/lib/chord_diagrams/fingerings.rb