Create continuation measures

[jaredjj3]

I expect a lot of users to consume vexml renderings using a mobile device. It's possible for a single measure to not fit in the viewport. I'm requesting a mechanism to detect when a measure is too big, and then to fragment it until it fits. Each fragment would display in a different system with obvious indicators that the measure is being continued.

There are two main parts to this project:

• We need an algorithm to fragment a measure. Since width is the constraint, my vote would be to find the column of notes that's approximately in the middle. After that, fragment using the measure beat. If there are already fragments, maybe follow the same algorithm on the fragment that's too big.
• We need to update the rendering hierarchy to behave well with measures that span multiple systems. At the time of writing, Refactor vexml for release #256 is still in flight, so I'm not sure where this will land.