plugin: grids

This new plugin (along with grids.css) can be called to generate a css grid configuration on an element and its children. Currently, we only have 2 grids: a (50/50) and b (33/33/33), but this will need to grow as other plugins use them (and they'll likely change depending on resolution as well). Globalnav uses this plugin in both its tab navigation and grid view. We should make this available through data-grid="a/b/etc" attribute too I think...