- Adds a tag plugin that allows more easily tagging pages.

  - Adds a tag plugin that allows more easily tagging pages.
    The meta plugin can also still be used for that, but the tag plugin
    also lists a page's tags at the bottom of the page.
  - Allows preprocessor directives to have parameters with no specified
    value.
  - Fixes preprocessor directive parameter parsing so that
    foo=bar baz now means "foo=bar" and a "baz" with no value.
* Run pagetemplate hooks when inlining pages so that inlines pages also
  list their tags.
* Make all plugins with pagetemplate hooks check that variables exist
  on the template before setting them.