Enhanced Footnotes

There is a request for enhanced, GitHub-style footnotes, which I would also like for my personal site.

I have implemented a change to Zola such that, when markdown.bottom_footnotes is false Zola renders footnotes in the “old” style, but when it is true, it uses the GitHub style.

I followed the implementation by notriddle.

GitHub told me to discuss here before opening the PR. Let me know if you need anything more before I open the PR.