Adds style linting like we do it in Gutenberg

[scruffian]

Changes proposed in this Pull Request:

This adds a stylelint config and also adds stylelint to the pre commit hook so we catch and fix issues in our Sass files.

To test, try committing a change to sass file. You should see warnings about issues in only that file.

[pbking]

This worked for me and I recommend going forward with bringing it in.

My only suggestion is to remove the lint-css and (especially) the lint-css:fix scripts. The fix in-particular, in my opinion, should never ever be ran. Ensuring everything henceforth is quality sounds great. Fixing everything in the past means one very large commit of questionable value.

[scruffian]

Good call. They were there for testing :)