Add css grid support

[Pajn]

The properties gridColumnStart, gridColumnEnd, gridRowStart and gridRowEnd takes numbers but currently glamor suffixes these numbers with px which causes invalid CSS. glamor even adds px when I pass the numbers in a string which makes these properties impossible to use with glamor.

[ChristopherBiscardi]

you can also name the lines and use those names instead of the numbers as a workaround.

grid-template-columns: [first] 40px [line2] 50px;