Add bubble color to the controller

The controller managing the documentation has now a property to add colors to the doc's bubbles. I added it there to have just one place to modify it.

I also simplified the bubble's template a little bit.

On the other hand, if the bubble's directive is now a little bit verbose, we could eliminate the `colors` attribute or leave it as true by default. But that is another history.

This fixes #45 and needs balderdashy/sails-docs#260 to be merged as well.