Improve footnote rendering

[CameronNemo]

A while back some folks brought up the footnote rendering in Zola and some ways to improve it. At the time it was determined to be a limitation of pulldown-cmark and the issue was closed (#1070). However, I seem to have found that in the meantime pulldown-cmark has offered some improved rendering options for footnotes, however the consumer has to make some effort to use the improved rendering options. See this comment for more details. It seems like zola just needs to integrate something like the example provided by pulldown-cmark.