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, allowing users to easily toggle these options in global.scss. This gives much more control over what people want in their outputted CSS and could save a lot of reusable code.