Theme configuration

Requirements
At a stark minimum, we need themes to be able to specify a base theme style to pull in; many themes will pull the base style at least (but some won't want it), and some may wish to just amend an existing theme.

File format
'theme.ini' probably good enough?

Example
?

name=Victorian author=webmaven link=http://example.com/ include=base

In current master (0.9.5) only the include parameter is implemented.

Include implementation
Check presence of any 'include' entries and pull in their .css files before the main one
 * should do dependency ordering? :D
 * it'll follow the chain down until something doesn't list an include dependency, then fetch their display.css files in reverse order so overrides work as expected

Other thoughts

 * metadata like authorship, name, link home?
 * specify multiple background images, color sets to present in admin & user options?
 * for things that exist in user or admin options but don't really apply to everything (#wrap, #aside_primary) maybe adjust things?