Make image references work
Make image references work

Images can be references, not just links. At this point, all major
functionality is implemented, what remains is bringing it into full
compliance with the spec.

500/548 tests passed

en90/pulldown-cmark,Eljay/pulldown-cmark,notriddle/pulldown-cmark,brson/pulldown-cmark,tempbottle/pulldown-cmark