Add default directive to top-level SASS variable definitions.

Added the `!default` directive to top-level variables. These can be easily overridden. As the kit is strongly used with the BEM naming convention, it would probably be worth re-factoring the top-level variables with more centric naming conventions, as mentioned, along the lines of BEM -- Block, Element, Modifier. 

Something like:

    // grid.scss
    // Define flag to decide whether or not to compile grids.scss to CSS.
    $grid--enable: false !default;

    $col__medium--count: 3 !default;
    $col__medium--width: 30.3% !default;
    $col__medium--gutter: 4.5% !default;

    $col__wide--count: 4 !default;
    $col__wide--width: 22.2% !default;
    $col__wide--gutter: 3.7% !default;

    @if ( $grid--enable == true ) {
        ...
    }

Then in global.scss, allow users to define modular SASS:

    $grid--enable: true;
    @import "components/grids";

Though, this could be purely included by default (eg. the `$grid--enable = true;` flag should be in `global.scss` to act as a switch), allowing users to easily toggle these options in `global.scss`. This gives much more control and scalability over what people want in their outputted CSS and could save a lot of reusable code. 

This also maintains the difference between design and layout.