Add the ability to specify track min and max sizes explicitly

[shyndman]

The improvements to the layout algorithm mean that allowing a user to explicitly set the min and max size for a track should be possible (similarly to how CSS' minmax() works).

In case I forget, here's an important paragraph for the track sizing algorithm.

If the track has an auto min track sizing function and the grid container is being sized under a min-/max-content constraint, set the track’s base size to the maximum of its items’ limited min-/max-content contributions (respectively), floored at zero. The limited min-/max-content contribution of an item is (for this purpose) its min-/max-content contribution (accordingly), limited by the max track sizing function (which could be the argument to a fit-content() track sizing function) if that is fixed and ultimately floored by its minimum contribution (defined below).

-https://www.w3.org/TR/css-grid-1/#algo-single-span-items