Add configuration vars for vendor prefix output

This allows a user to define globally what vendor prefixes will be generated.

A simple use case is if you are building an html iOS app only targeting the
`webkit` browser, you can declare these settings prior to calling any mixins
and tune down the final output. 

The variable naming mirrors what is found in [Compass](https://github.com/chriseppstein/compass/blob/stable/frameworks/compass/stylesheets/compass/_support.scss), which would 
hopefully ease the pain in someone transitioning over to Bourbon.