* Allow plugins to add new types of tests that can be used in PageSpecs.

* Allow plugins to add new types of tests that can be used in PageSpecs.
* Add a "conditional" plugin, which allows displaying text if a condition
  is true. It is enabled by default so conditional can be used in the
  basewiki.
* Use conditionals in the template for plugins, so that plugin pages
  say if they're currently enabled or not, and in various other places
  in the wiki.