Make it possible to take left-over space

[Andrea Giammarchi]

There are cases where

a b c
a d c

is meant to have central part of the grid taking over space ... the current solution is to use:

a b b b c
a d d d c
a d d d c
a d d d c

and while this works well both as ASCII representation and as final layout, it doesn't necessarily indicates that neither b nor d should take left-over place as they would do with grid-template-columns: 150px 1fr 150px or grid-template-rows: 150px 1fr.

It's always possible to add these details in the layout of the custom element as part of the style, but maybe it's better to find a way to automagically define these ?