Add note length

[davetapley]

We're trying to display tunes in a time signature (since #1), but we need note length.

I propose all notes are implicitly semibreves, but we may prefix a note with -. This note would be handled as normal, but could be used as a placeholder to indicate that two notes actually form a breve, thus:

C4 C4, is two semibreves.
C4 -C4 is a breve.