Make each generated page’s footnotes unique.

If content pages are fully rendered in a list page, footnotes that use the
same reference (`[^fn]`) will have duplicated anchors. This change builds
on #526 to put the page filename (`Page.File.Name`) as part of the anchor
for a footnote.

This would fix discussion
[116](http://discuss.gohugo.io/t/footnote-references-are-duplicated-on-list-pages/116).