Add footnotes

[hugomardbrink]

Add support for footnotes in latex and html.

[adelhult]

There are multiple ways we could go about implementing footnotes for the HTML format:

• Using a footnotes variable and for each [note] push a unique id to that list together with the content. Then let the [note] module spawn another [note-label] that reads the list after every item is added.
• Using a footnotes variable and changing the scheduling system to allow us to read the list while pushing and generating the label straight away.
• Generating some content in-place: like these articles do.
• Using "side-notes", see article. I think that might be the nicest one but I'm not really sure how we would go about implementing that with our current system.

After sleeping on it seems like the third approach might gives us the best result for the least effort. So I'll continue where Hugo left of yesterday and attempt at implementing that!

Nevermind, looks like <details> can't be used inside paragraphs. Back to square one : )