Add css commenting snippets for Sublime Text.

Add some Sublime Text snippets for helping people write CSS comments according to our commenting standards.

Status: **Ready to merge**

Reviewers: **@jeffkamo, @kpeatt **

## Changes
- Add css/editor-snippets directory
- Add sublime-text sub-dir
- Add Sass h1-style comment, tab trigger `//#`
- Add Sass h2-style comment, tab trigger `//##`
- Add Sass function docblock, tab trigger `//fn`
- Add Sass mixin docblock, tab trigger `//mx`
- All snippets are fielded, which means after adding them the developer can fill out the details by typing and tabbing to the next field.

## How to test-drive this PR

Note these will work with Sublime Text 2 as well, but easier to write instructions for one version.

- Get a copy of Sublime Text 3
- Copy the snippets to `~/Library/Application Support/Sublime Text 3/Packages/User/Snippets` (you may need to create the Snippets dir)
- Open a new file in Sublime
- Type each of the tab triggers listed above and press tab, then fill out the fields by typing and tabbing.

## Research resources
- http://sublimetext.info/docs/en/extensibility/snippets.html