Refine css style order, add tooling configuration

(Note: this re-creates #8)

This PR does two things: 
1. Proposes some small modifications to our preferred order of 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 (via plugin) to automatically sort properties in the correct order.

Status: **Ready for merge**

Reviewers: @kpeatt @jeffkamo @cole-sanderson @scalvert 
Ticket(s): N/A
Linked PRs: N/A

## Changes
- Updates the preferred order for authoring CSS rules;
- Adds missing properties (e.g. latest flexbox spec, some other obscure ones);
- Removes any vendor-prefixed properties that have a standards version (since autoprefixer will take care of that);
- Adds a .scss-lint.yml config file for use with the scss-lint tool;
- Adds a .csscomb.json config for use with the css-comb tool;

The scss-lint config will work with our existing linting tasks. CSS comb is new, and will allow us to automate rule re-ordering via an editor plugin (available for Sublime, Atom, Jetbrains, Vim, possibly others).

## 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)