Add HTML tag removal for footnotes

[scherr]

The existing RemoveHtmlTags converter does not handle footnotes:

doc = Kramdown::Document.new("Test[^1]\n\n[^1]: <script>alert(1);</script>")
Kramdown::Converter::RemoveHtmlTags.convert(doc.root, :remove_block_html_tags => true, :remove_span_html_tags => true)

Kramdown::Converter::Kramdown.convert(doc.root)
# => ["Test[^1]\n\n[^1]:\n    <script>alert(1);</script>\n\n", []]

Kramdown::Converter::Html.convert(doc.root)
# => ["<p>Test<sup id=\"fnref:1\"><a href=\"#fn:1\" class=\"footnote\">1</a></sup></p>\n\n<div class=\"footnotes\">\n  <ol>\n    <li id=\"fn:1\">\n      <script>alert(1);</script>\n\n      <p><a href=\"#fnref:1\" class=\"reversefootnote\">&#8617;</a></p>\n    </li>\n  </ol>\n</div>\n", []]

This PR attempts to address this. However, it's quite possible that there is a more sensible way to cover this.

[gettalong]

Thanks for the pull request - I will have some time this weekend to see if your change covers everything.

[gettalong]

I have merged your commit and amended it to also return the correct value in case of el.type == :footnote. Will be in the next release.

[scherr]

Ah, great. I had missed that. 👍