Handle math

[deepakjois]

This might turn out to be really difficult, but as a start maybe we can provide a workable fallback if we are typesetting math stuff.

[khaledhosny]

I think it shouldn’t be hard to populate the math related fields of the font metrics using HarfBuzz API and then let LuaTeX do the actual typesetting since this is handled by the engine itself and it has rather decent support for OpenType math layout.