Make Octopress only load its configs once per process.

Rebased against current 2.1 branch, and resubmitted.

A request: Please make a call on this and either land/reject/kick-it-back-for-changes before it requires rebasing again. This is getting awfully painful -- roughly 3/4 of my time spent contributing to Octopress has been spent around process overhead, rather than useful work, and that's not including things like going back and making test cases and such.  >.<