autolink headers

(I'm coding this on top of n8han/pamflet#40)

This adds autolinking of headers. pamflet already extends knockoff to add `id` on headers.
When the user hovers over a header, it will now display '§' after the header that links to the header itself.
See http://eed3si9n.com/tetrix-in-scala/specs2.html#bdd for example.