Option to exclude default theme

I include a theme provided by http://bootswatch.com/ but the distributed Bootstrap theme that is imported by this addon causes problems. All I need to do is exclude the default Bootstrap theme. This PR adds the option `excludeBootstrapTheme` (named similarly to the JS option) and provides backwards compatibility with its default of `false`.

Looking at PR #14 there might be some need for granularity. So, I'm throwing this out there but more thought is probably needed. I'm willing to discuss other options and create PR's.