Support :FOOTNOTES option for rendering HTML

[aharpole]

This adds the ability to render Markdown with footnotes into HTML.

Example:

markdown = <<~MARKDOWN
  This Markdown has footnotes[^1]!

  [^1]: Pretty neat
  MARKDOWN
CommonMarker.render_html(markdown, :FOOTNOTES)
=> "<p>This Markdown has footnotes<sup class=\"footnote-ref\"><a href=\"#fn1\" id=\"fnref1\">1</a></sup>!</p>\n<section class=\"footnotes\">\n<ol>\n<li id=\"fn1\">\n<p>Pretty neat <a href=\"#fnref1\" class=\"footnote-backref\">↩</a></p>\n</li>\n</ol>\n</section>\n"

[gjtorikian]

Thanks very much for this! A few things:

1. Would you mind updating the README as well?
2. I hadn't noticed this still used Travis/Appveyor. In Switch to GitHub Actions #132 I migrated over to GitHub Actions; could you pull in main to trigger that rebuild?
3. If GitHub / Microsoft are relying on this project, please consider sponsoring @kivikakk and myself to maintain it. Thank you.

[aharpole]

Thank you, @gjtorikian, and I will talk to the powers that be to see about sponsorship. Thank you for your efforts maintaining this!

[kivikakk]

commonmarker 0.21.2 has been released with this functionality in place!

$ gem install commonmarker
Building native extensions. This could take a while...
Successfully installed commonmarker-0.21.2
Parsing documentation for commonmarker-0.21.2
Installing ri documentation for commonmarker-0.21.2
Done installing documentation for commonmarker after 0 seconds
1 gem installed
$ irb -rcommonmarker
irb(main):001:0> markdown = <<~MARKDOWN
irb(main):002:0"   This Markdown has footnotes[^1]!
irb(main):003:0"
irb(main):004:0"   [^1]: Pretty neat
irb(main):005:0"   MARKDOWN
=> "This Markdown has footnotes[^1]!\n\n[^1]: Pretty neat\n"
irb(main):006:0> CommonMarker.render_html(markdown, :FOOTNOTES)
=> "<p>This Markdown has footnotes<sup class=\"footnote-ref\"><a href=\"#fn1\" id=\"fnref1\">1</a></sup>!</p>\n<section class=\"footnotes\">\n<ol>\n<li id=\"fn1\">\n<p>Pretty neat <a href=\"#fnref1\" class=\"footnote-backref\">↩</a></p>\n</li>\n</ol>\n</section>\n"
irb(main):007:0>