Drop divs, use footnotes instead

[rwxrob]

So, Pandoc divs are non-standard and not supported pretty much anywhere but Pandoc. So I'm dropping them from KEGML in favor of the one true way to make annotations: footnotes. This way there is absolutely nothing in the readers way when reading and they can choose to look it up if they like. This is the way "admonitions" and "notes" and such have been done for more than three decades and there is really no reason to change that.

Anything longer than a paragraph in a footnote deserves its own content node anyway.