Modify chord handling to handle guitar capos

Modify chord handling to handle guitar capos

chord-name-engraver now calls a shim between it and the chord formatter
in order to print capo chords

chord-name-scm has had capo-handler added. If requested it prints both
the original chord, and the transposed chord.

define-context-properties adds the properties capoPitch to declare the
capo transposition, and capoVertical to make the chords print in column
rather than row form.