Fix #637 : Markdown footnotes

[papey]

Here is fix for issue #637 that :

• Ensure footnotes classes are not filtered by html sanitizer
• Add some footnote style rules

An example of footnotes using this fix :

Markdown :

Here is something[^a]


Here is another thing[^b]

[^a]: something

[^b]: another thing

Note that there is some cpull-markdown related syntax : a carriage return is needed between each note definition

[papey]

Note that back reference are not supported (yet) by cpull-markdown : pulldown-cmark/pulldown-cmark#142

[codecov]

Codecov Report

Merging #700 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #700   +/-   ##
=======================================
  Coverage   39.25%   39.25%           
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  Files          72       72           
  Lines        9498     9498           
  Branches     2265     2265           
=======================================
  Hits         3728     3728           
  Misses       4713     4713           
  Partials     1057     1057

[elegaanz]

Thanks!