Handle ties

[alexanderkoller]

The ABC file in #7 uses the minus character (-), but I can't figure out what it is supposed to mean. Change the AbcParser to gracefully skip (and report) unsupported characters, instead of simply crashing.

[alexanderkoller]

The minus character is for ties. The new parser recognizes these now, but the AbcParser ignores them. We should handle them correctly at some point.