Footnotes now supported by GitHub's web interface

[rokejulianlockhart]

@samwilson, see:

Footnotes and indentation

While trying to implement footnotes, what would be the best indentation rules for continuation? Should it be done a bit similar to the way CommonMark is already working for list? PHP Markdown Extra and Pandoc for example require a fixed indentation of a tab, but in the philosophy of CommonMark shouldn’t we indent on the first non-whitespace char following the [^xxx]: This is a text[^1] [^1]: This is footnote > This is quote block Tis is not a continuation