Rather than offset/width, use a single adjust field

[It has been pointed out](https://github.com/guideguide/support/issues/11) that there isn't a reliable way to center a grid. This alters the options spec to allow for a grid-like option notation for adjustments in a single value, rather than many values together.

Centered, 100px wide grid `~|100px|~`

Centered grid: `~|~`

Right aligned grid: `~|`