Removed `rev="footnote"` from HTML output.
Removed `rev="footnote"` from HTML output.

The `rel` attribute was removed a while ago, but for some reason `rev` stayed. It shouldn’t.

mariokonrad/php-markdown,ashur/php-markdown,filipstefansson/bootstrap-3-shortcodes,jockchou/php-markdown,MWDelaney/bootstrap-3-shortcodes,Kapeli/php-markdown,MWDelaney/bootstrap-3-shortcodes,filipstefansson/bootstrap-3-shortcodes,robbieaverill/php-markdown,n054/php-markdown,queirozfcom/php-markdown-with-syntax-highlight,MWDelaney/bootstrap-3-shortcodes