[css-grid] Varying-width Grids

This is one of the complaints about flexbox:
   https://twitter.com/AlanHogan/status/519256635911307265/photo/1

It's really more of a grid-layout problem, but we can't actually
solve it in CSS Grid Layout. Probably something that should be
fixed in this level, as it's quite common.

Constraints as far as I can tell:
   * Size grid columns to >= multiples of X.
   * Flex gutters equally to fill the width of the container.
   * Auto-fill grid.

We could probably leverage 'justify-content' to deal with
the second point. Auto-fill is already there. The main issue
is the first item. Perhaps it's something we can do with a
repeat() function?

Post ideas here. :)

~fantasai