Add a "plain" preset that is just "bootstrap" with no extra styles.

This is useful for theme designers wanting to customize without having to undo extra overrides from the presets.

Rationale (courtesy of Damyon):

My reason for this preset is that the "default" preset is bootstrap + a bunch of styles and other changes that we think look good, but probably some people don't like. If they are used to styling things for bootstrap, and they use the "default" preset + the scss settings on the advanced tab - the first thing they need to do is start "undoing" our extra changes from our default preset.