Rewrite colors into shortest possible name

Improve the code for rewriting colors into recognising that some
colors are shorter by name.  This commit enables scour to rewrite
`rgb(255, 0, 0)` into `red` which is slightly shorter than `#f00`
(ditto for `tan` and `pink`).

When the color name ties in length with the hexcode, then scour will
leave it as-is if the input file used a variant of same length
(e.g. `blue`, `cyan` and `aqua` will be left as-is).  But if scour is
rewriting the color code, it will prefer the hex code variant.

Signed-off-by: Niels Thykier <niels@thykier.net>