Allow generating and using external CSS files

Originally created by: fritzophrenic

Original report by Ben Fritz (Bitbucket: fritzophrenic NA, ).

We should have a ToCSS command to convert the current colorscheme into a standalone CSS stylesheet, and an option for TOhtml that will use such files.

Doing this properly will also require options to TOhtml for unlinking highlight groups, so that color schemes can support drop-in replacement. Probably provide two levels for this: one for unlinking only the "top-level" names like "Comment" or "Identifier", and another level for unlinking all names, since some color schemes actually define colors for specific syntax items for certain file types.

I received a patch for an initial stab at a ToCSS command at some point, I'll attach that to this issue sometime in the future if it remains open too long or if someone wants to work it.