Make PedalMarking boundaries configurable

[jaredjj3]

In https://github.com/stringsync/vexml/pull/209, I'm trying to render PedalMarking objects that should span the entire measure.

The PedalMarking objects are associated with the correct notes, but I'm relegated to only be able to render markings at the beginning of the note.

I'm requesting for a way to specify the note edges that the PedalMarking boundaries should render.