Added explicit named links for TOC

Using redcarpet means we lose the 'auto-ids' feature of kramdown, to gain syntax highlighting. To compensate, I've put anchors next to the headings.

It seems to be impossible to have both auto-ids and syntax highlighting -- kramdown (the default) supports the former but breaks the latter, and redcarpet doesn't have the former but makes the latter work trivially.

Oh by the way, gh-pages disables plugins, so this can't be corrected with e.g., kramdown-with-pygments or whatever.

There are at least six ruby implementations of markdown. Don't you people have better things to do?