Update default config

* Adds upcoming [rule-delimiter](https://github.com/csscomb/csscomb.js/issues/210) option. Will keep one linespace space between each ruleset.
* Adds [sort-order-fallback](https://github.com/csscomb/csscomb.js/blob/master/doc/options.md#sort-order-fallback) option for properties not explicitly defined.
* Adds [space-between-declarations](https://github.com/csscomb/csscomb.js/blob/master/doc/options.md#space-between-declarations) option to keep each declaration (`property: value;`) on separate lines.
* Makes SASS features appear with a clear linespace before CSS properties. Only applicable when combing .scss.
* Adds `src` property, so that combing `@font-face` ruleset doesn't result in .eot always coming last and breaking fonts in Firefox.
* Adds `font-smoothing`, `backface-visibility` and `text-rendering` properties with vendor prefixes as needed.

Diff best seen with `?w=0` to ignore whitespace changes: https://github.com/GaryJones/grunt-wp-css/commit/0887c06993c7eeea2c716d2d9ba523f3663dfa8b?w=0