Add support for customizing navigation bar styles in the MkDocs theme

[jimporter]

See also: mkdocs/mkdocs-bootswatch#56

This patch allows documentation authors to configure the color of the navigation bar with one of three options (defined by the Bootswatch theme): primary (the default), dark, and light. This is primarily here to restore support for (what used to be called) inverse nav headers in mkdocs-bootswatch. Without this patch, mkdocs-bootswatch would have to redefine all of base.html for each theme to include the changes made here. Since this feature is relevant for the mkdocs theme, I figured it made more sense to add it in the base theme.

The Jinja template change is a bit hairy here, but I'm not sure if there's a better way to do it...

Here are some examples of what it looks like with the mkdocs theme:

primary

dark

light

[waylan]

Cool. Didn't realize this was an option bootstrap offered.

Add a note to the release notes and I'll merge this.