Create a whitelist of sass opts in safe mode.

This PR essentially locks down the entire configuration process in safe mode such that, well, nothing custom can be set. If we want to allow users to set any of the configuration options, we can do that, too. But for now, it leaves everything to default except `cache`, `load_paths`, and `syntax`.

There is a huge list of [Sass configuration options](http://sass-lang.com/documentation/file.SASS_REFERENCE.html#options).

/cc @mastahyeti @benbalter