Implement shortcut for inserting multiple stropha

[frmatthew]

e.g., gabc says that hsss should in fact create three notes of the stropha form.

Shouldn't be too hard: if we get an s note modifier, then change note type to a stropha. If there's a second s modifier, duplicate the note, a third, duplicate again, etc.

Do we need to be concerned about different markings on different repititions?

[frmatthew]

Also, multiple v's to create multiple virgas. See https://github.com/gregorio-project/gregorio-test/blob/master/tests/gabc-dump/glyphs/repercussion.pdf