Overhaul config

* defaults centrally located
* documentation centrally located
* basic infrastructure for config value validation
* rewrote lots of config variable related documentation
* fixed test harness to build datadir and themedir correctly
Remove last vestiges of comments plugin
Make base_url required; no more composition

* nixes all the places where we were composing base_url or
  defaulting it to ''; it's now a required configuration parameter
* fixes some cases where we had config defaults for other variables
  floating around

Fixes #32