initial implementation of footnotes extension

[3b]

inline references like [^foo], with definitions as blocks starting with [^foo]:.

questions for users::

• bugs/missing features?
• what should be configurable and/or have different defaults?
  • css classes?
  • formatting of links?
  • container element for list of footnotes?
  • separator before/after definitions?
• is ^[inline footnote] version common/useful enough to be worth implementing? (or some other syntax)

todo:

• text/md output
• documentation
• configuration
• tests
• improve formatting of output?
• option for adding some extra text to footnote IDs to allow displaying multiple markdown documents on one page (for example comments, or making an html file out of the test suite)