Add PHP-Markdown style footnotes support

@djui opened an [identical pull request](https://github.com/vmg/redcarpet/pull/241) a month ago, but it has stalled. Thought I would submit my own with the requested changes.

This includes the great original work by @bdolman and @adamflorin, a fix to footnote parsing by @microjo, a test case in `test/html_render_test.rb`, and edits to the changelog and readme.