Add footnote parsing/rendering support
Add footnote parsing/rendering support

Fixes GitbookIO/gitbook#395, fixes GitbookIO/gitbook#134, fixes
GitbookIO/gitbook#237

Based off https://github.com/chjj/marked/pull/431

emilbayes/kramed,GitbookIO/kramed,gdbooks/kramed,martindale/kramed,Ponyshow/pony-render,GitbookIO/kramed,emilbayes/kramed,GerHobbelt/kramed,GerHobbelt/kramed,martindale/kramed,gdbooks/kramed