This patch adds color highlighting to CSS mode.

In particular, this provides a new jit-lock function that recognizes the
various forms of CSS color syntax, including some css-color-4 additions.

When such a color is seen, the background color of the text is set to
the color itself.  In order to remain readable, the foreground is set to
a contrasting color, and a box is put around the text (this helps
distinguish colors that are close to the buffer's background color).

While doing this I noticed that css-mode was missing a few named colors
specified by CSS, in particular:

    ("darkgrey" "darkslategrey" "dimgrey" "grey" "lightgrey"
    "lightslategrey" "slategrey")

This patch fixes this problem as well.