Conform footnote handling to PHP v4

Textile JS adds backlinks to footnotes by default (as does RedCloth) while PHP does not. PHP uses an alternate syntax to trigger backlinks. For compatibility, the following needs to be done:

* [x] Turn auto-backlinks off by default (and provide a setting to turn the behaviour on/off)
* [x] Support backlink syntax in footnote blocks: `fn1^. TEXT`
