'grid-area' now is a third mode, neither flow nor abspos.

Slot names are identifiers instead of letters.
The slot names may be quoted, and must be quoted if they are keywords.
Put the column widths before the template in the 'grid' shorthand.
Some improvements to text.
Added note about field/module terminology.
Added example showing white space issues.
'Grid-area' syntax simplied (only width height, no bottom and right coords).

Author: bert-github