Footnotes are not supported in Markdown

[shenxianpeng]

I used footnotes in the project README, which looks like the following.

click [1] to jump to the footnotes

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Hower on https://pypi.org/project/gitstats, it will display like this which does not support footnotes

[miketheman]

This is a CommonMark extension, not enabled by default, and would need to have CMARK_OPT_FOOTNOTES enabled to work.

The way to solve is to add a test fixture test_GFM_footnotes.md and corresponding test_GFM_footnotes.html with the correct rendered output, and then update markdown.py to pass the option.

[shenxianpeng]

I have opened a pull request #324 to address this issue. Appreciate your review and feedback.