Add support for footnotes

This updates the sundown submodule with footnote support and merges/adopts these changed with redcarpet.

Please note that credits go to a couple of people which I can't even track anymore: one definitively being @adamflorin and @bdolman . I was more an orchestrator <- no tests ;-(

It seems to be pretty stable as @bdolman reported which is used in a couple of websites successfully.