Add SassDoc syntax extension for Sass

This additional component highlights SassDoc comments in Sass/Scss code.

The code is from <https://github.com/SassDoc/prism-scss-sassdoc>.

Note: the `.min.js` file has bin generated with uglify-js 2.4.15, I
don't know the tool and options you're usually using, and there's no
makefile or something to generate the minified files.