Add feature to support use of CSS Grid / flexbox

[notlee]

This is kind of a vague feature request. Many people use o-grid on new FT projects where flexbox or CSS grid would be a better choice. Origami don't provide any tools to help users match FT styles with flexbox or CSS grid however. For example o-grid defines a different gap depending on viewport size, even a mixin which output this as a gap value could be helpful. But this is an open ended issue... what happens to o-grid now we have CSS grid and flexbox?