Add permalinks to post headers

A quick patch to the Redcarpet renderer that creates headers with permalink
anchors. There is one ugly bit about this: it shows up when post content is
rendered outside of post pages.

For example, `index.html` should never show post permalinks because they aren't
permanent --- they will change as the content scrolls off the page.

Also, added a Gemfile that pins the Redcarpet version so that I don't wonder
why my plugin code is breaking the next time Redcarpet has a version bump.