Pasting Markdown-with-math (code available)

[inducer]

I've cobbled together a solution that seems to do a satisfactory job of allowing pasted Markdown, see this PR to my project:

https://github.com/inducer/relate/pull/1148/files

It's certainly less broad in scope than #17 (but perhaps a bit more ambitious than #18?). This could probably be chopped out and moved into prosemirror-math.