Added default flag to customizable grid vars

This change makes grid vars that are intended to be customizable per-project overridable outside of this file by adding the [`!default` flag](http://j.mp/1q4MAiK) to them. This means users don't need to modify this file when they want custom values for their grid; instead, they can just define those variables in their Sass prior to importing `grid.scss` as a partial.