Accidentals (flats) ignored

Accidentals are the one case (at least that I'm aware of) where a single glyph's presence doesn't create an MEI element of its own (as with neume components, custos, clefs, etc); its presence is registered by adding an attribute to the _following_ glyph. This'll require special treatment in the MEI building routine, though I'm not sure where to put it yet; trying to avoid turning it into a tower of `if` statements.