Request: Add round and space to background-repeat

It would be great if you could add both `background-repeat: round` and `background-repeat: space` to the background reference since they're really useful in some scenarios.

Links:
https://css-tricks.com/almanac/properties/b/background-repeat/
http://caniuse.com/#feat=background-repeat-round-space
http://cssvalues.com/#background-repeat