Bug 738505 - Add fontfeatures support in PangoAttributes and markup

https://bugzilla.gnome.org/show_bug.cgi?id=738505

Patch from Matthias Clasen, based on early patch from Akira TAGOH.

There's room for improvement in how this is done, but it works now
for simple cases, which is what most people will be using it for.

Finally!