aligning at marker *across* equations

Just to track something that came up one the DPUG IG,

Use case: authors want to align equations at markers across math elements (or equivalent), cf. http://www.w3.org/TR/dpub-latinreq/#aligning-equations

Tentatively marking this MML spec limitation even though it's perhaps more HTML5 spec limitation.

Adding it to v3.0 milestone so that we think about it then ;-)