Add support for Sass and SCSS.
Fixes for Sass/SCSS converters.
Allow users to specify options for Sass.
Consolidate into one master 'Sass' converter.
Build Sass configuration options.
Don't allow path traversal or syntax overrides.
Does not allow caching.

This was done to prevent putting the .sass-cache folder in bad places. Needed? cc @benbalter.
Make sass a runtime dependency.
Add docs for Sass conversion.
Allow caching in unsafe mode, but disable in safe mode.
Better express that the converter always outputs css
Improve the test description for the outputs_ext test
Merge master into sass
fixes based on @mattr-'s notes
Merge pull request #1932 from jekyll/sass