dwconfig.py: improve no-caching option(s)

Two changes:
- --no-cache now disables the IMC as well as everything else, just in case.
- new --no-brute option to disable just the BFC and the IMC. This sort of
  duplicates testbench.py's -B option, but that's okay; it should be a
  general option anyways.

Arguably there should be general options to specify the cache directory
and to enable the renderer cache, but now I'm starting to wave my hands
vaguely.
httpcore.py: add an option to force the IMC on

This is intended for when you're running DWiki as a real WSGI application
under a preforking WSGI server such as uWSGI.
Update pages/dwiki/ConfigurationFile and GlobalVariables

ConfigurationFile now documents the new imc-force-on setting and also
some settings normally controlled by command line flags.

GlobalVariables now documents :wikitext:title:nohtml and has a somewhat
better discussion about how to get the :wikitext variables set.
README: add stuff about running DWiki as a real WSGI application

Since I've now done this for testing I can actually write about
it, so I should.