Create settings to manage which responsive breakpoints are active.

The idea is to set a few settings in our variables files that define which breakpoints are active and which are not. In our variables file, we can add something like

```
$use-breakpoint-lg:     true !default
$use-breakpoint-md:   true !default
$use-breakpoint-sml:  true !default
$use-breakpoint-xs:    true !default
```

Then in our grid file, we can can activate our breakpoints accordingly.
