Auto merge of #327 - tiaanl:add-none-keyword, r=emilio

Add none keyword

For each color component, we allow the "none" keyword.  The keyword will be kept in place for serialization.  The "none" keyword is not allowed in the legacy syntax.

`rgb` and `hsl` are handled special, because they support the legacy color syntax.  They also cannot be combined, because rgb cares about the type of the components, and hsl does not.

Author: bors-servo