Update preferred CSS property order, add tooling config

This PR does two things: 
1. Proposes some small modifications to the preferred order for writing CSS properties;
2. Adds tooling configuration for scss-lint and csscomb to match 1).

The scss-lint config is our primary linter, which we had previously included in the old project scaffold and should now move to this repo. It would be a great idea to integrate this with our project generator and Grunt.

Although linting CSS is great, having to think about CSS rule order is a mental drain. That’s where CSS Comb comes in: it provides editor commands (for supported editors) to automatically sort properties in the correct order.

Status: **Opened for visibility**

Reviewers: @kpeatt @jeffkamo @cole-sanderson @scalvert 
Ticket(s): N/A
Linked PRs: N/A

## Changes
- Updates the preferred order for authoring CSS rules;
- Adds a .scss-lint.yml config file for use with the scss-lint command-line tool and/or Grunt.
- Adds a .csscomb.json config for automating rule re-ordering in supported editors (including Sublime).

## How to test-drive this PR
- Review the diff or read through the Readme until the end of the property sorting example
- You can also compare with the [current scss-lint config](https://github.com/mobify/scaffold/blob/master/mobify.scss-lint.yml)