Enable layout with a variable

I've ran into a situation where I'd like to make changes to the layout system's gutters based on a media query; however, I'd still like to keep the modifiers (`.layout--flush`, `.layout--small`, etc.) intact. 

My initial implementation was to put my media query changes **after** importing the `objects.layout` file. This was problematic because my media query changes overrode the modifier classes. Similarly, if I put the media query changes **before** importing the file, the default `.layout` negative margin would override my changes.

It seems the best solution would be to allow the option to include both the default `.layout` classes as well as any of the modifying classes simply with variables. Thus, I could achieve my goal like so:

```scss
// Import the base `.layout` and `.layout__item` classes
@import "bower_components/inuit-layout/objects.layout";

@include media-query(palm) {
  // Make my media query changes to the gutters
}

// Import the rest of the modifier classes but **without** the base classes again.
$inuit-enable-layout: false;
$inuit-enable-layout--small: true;
$inuit-enable-layout--large: true;
$inuit-enable-layout--flush: true;
@import "bower_components/inuit-layout/objects.layout";
```