Fixes #56. Adds support for footnotes to cmark-gfm. This is a key requirement of being able to be used where Kramdown is; for instance, Kramdown is the default Markdown renderer in GitHub Pages.

This PR does the following:

• Generalises the map used for reference link lookups to support both reference links and footnotes.
  • It's still fairly specific to the needs of both (it's not a "generalised map" by any means), but that's fine.
• Parses footnote definitions (block-level) and references (inline-level) according to kramdown's syntax.
• At the parsing finalisation stage:
  • Collects definitions in one pass over the document and added to a map.
  • Collects references in another pass, assigning increasing indexes to the definitions so-referenced. (i.e. footnote #​1 is the first footnote referenced, not the first defined.)
  • Adds definitions that were referenced to the end of the AST in reference order.
• The HTML formatter produces links and footnote sections according to Kramdown's output format (for now).
• The CommonMark formatter roundtrips footnotes correctly.

To-do:

• reduce duplication between reference map/footnote map
• don't use linear search for footnote definitions when finalizing
• fix the static unsigned int ix; hack in commonmark.c whatever it's fine
• take our snprintf results and turn them into asserts
• make footnote parsing optional (it costs a bit)