Add gutterless display option to span-columns

This adds a new display option `gutterless` to span-columns that uses a simple percentage calculation so that developers/designers have options on how they want to collapse gutters.

I added the `gutterless` option rather than changing `collapse` because a percentage calculation on `collapse` could have ill effects when someone attempts to mix collapse with the default display mode.

I assume that this will bring up discussion rather than be merged immediately since it is a new feature with added complexity.

Fixes #104