Add shortcut for SASS imports to look in node_modules

[cchaos]

Similar to the way that LESS is configured to load non-relative paths via the @import "~[library-name]" syntax, we'll want SASS to add node_modules to the searchable folders.

Based on talking with @tylersmalley we're using node-sass which looks to utilize this feature through https://www.npmjs.com/package/node-sass#includepaths