Options to include bootstrap's theme and exclude bootstrap base css

The base Bootstrap css is typically used, and the theme css can be layered on-top but this is considered optional on getbootstrap.com.  For this reason, the theme css is now excluded unless explicitly added.

On a similar note, this pull request includes work by @jkatsnelson who brings up a plausible use case where someone has custom built bootstrap.css using a sass or less variant.