Add columns prop

[sapegin]

To easily define number of columns in a responsive way, similar to Theme UI Grid component.

Note: columns may not be the best name, as it conflicts with the columns CSS property, and this is something I'd like to avoid in stack-styled.

Note 2: either columns or minColumnWidth can be used, but not both at the same time, so it would be great to show a warning when the user tries to use both props.