bug#25525: 25.1.90; add color highlighting to css mode

[Simen Heggestøyl]

Thanks Tom, this looks to be a very nice addition to CSS mode!

Some comments:

- It should be possible to disable it. I'm not sure whether it should
 be turned on by default or not, though I'd think that most CSS
 developers will find it useful.

- Selectors that happen to contain the name of a color are
 highlighted. The 'red' part of '.centered-button' is highlighted in
 red, for instance.

- The same goes for variable names in both CSS and SCSS.

- Property names are also highlighted, for instance in the case of the
 'white-space' property.

- There's a stray call to 'message' in the beginning of
 'css--fontify-colors'.

- The error message in 'css--compute-color' should be capitalized.

- I think it deserves a NEWS entry.

-- Simen