Ability to provide common ABC props

[chrisglein]

Perhaps we have a space for "common ABC props"?

Can preload with this kind of stuff:

[I: MIDI=program 20]
%%titlefont harrington 40
%%vocalfont harrington 10
%%gchordfont harrington 12
%%notecolors 1
%%notelabels 1

Actually, I think that's a good idea. I'll open an issue.

Originally posted by @chrisglein in #25 (comment)

[brewingcode]

@chrisglein Was this something that you were thinking would be:

1. editable in the UI, or
2. a handy place in Javascript to specify
3. specified in abc.js itself (or our fork of it)

[chrisglein]

I was thinking a place in the UI. Some shared settings across sheets, accessible when you want to through a hamburger menu or something. Hidden by default.