Add configurable Base Theme setting

Adds a setting to specify the base theme you want to use. No longer
do you have to use simple. Can be specified from your settings
file and the command line. The base theme directory is also
watched for changes so that autoreload works.

Of course I need to make some documentation updates but I wanted to get feedback first.

What are the thoughts on tests for this? I feel that the tests used to make sure content is generated correctly is enough. I haven't found the THEME setting to be tested in any other way (thought I could be wrong).

I advise that this not be merged until after the upcoming 3.4 release, just because its a large feature addition/commit. However, if no problems are found, I see no reason to hold it back.

This is an implementation what was requested in #1092