The attached patch adds completion for CSS color functions such as
`rgb()'. Values were taken from the “CSS Color Module Level 4”. It can
be found at: https://drafts.csswg.org/css-color/#color-type

As a sidenote, how should we handle functions? Since they use
parentheses, the point is going to be outside of parentheses.

Eg.
rgba()*
      ^(point)

Also, should we include the parameters number?