Colorize footnotes

[meisterluk]

Before

Footnotes are always rendered completely gray (#999999).

After

3 keys are added to the base theme, namely:

footnote_reference_font_color: '999999'
footnote_marker_font_color: '999999'
footnote_text_font_color: '999999'

which can be overridden by the user in a YAML theme file:

footnote:
  reference_font_color: #00FF00
  marker_font_color: #FF0000
  text_font_color: #0000FF

Theme extensions might require further considerations such as exchanging square bracket [] with Japanese 【】 or alike. But this patch implements a current usecase I have for German text. So this pull request is restricted to colorizing footnotes.

Feel free to discuss 😊

[mojavelinux]

I'm certain we'll need keys in the theme to control the appearance of footnotes, but right now the placement of the footnote text is temporary (just a placeholder). I'd rather spend the time working this into a proper solution for footnotes as described #73 (emitting them as endnotes at the end of the chapter for the book doctype and the end of the document for the article doctype). I'll continue the discussion there.

[mojavelinux]

I believe this will be resolved by the fix for #85. If the footnotes are displayed at the end of the chapter, and can be individually styled, do you still think it's necessary to be able to control the style of the footnote marker?

[mojavelinux]

Now that #85 has been resolved, I'm going to close this issue. If you'd like to still be able to customize settings which are currently hardcoded, feel free to submit another PR and I'll see that it gets merged.