This adds an option (always on at the moment) to use a colour scale of green to yellow to orange for the file size field instead of always green. See #65.

This has to be done for when ignore patterns get introduced and have to be stored in a Vec.

Now when you do `--sort time` instead of saying "unknown option --sort
time" it will say "unknown options '--sort time' (choices: name...)"
with all legal options.

This also adds the legal values to the default help text.

The original options was becoming a bit unwieldy, and would have been even more so if I added the same amount of comments. So this commit splits it up.

There's no extra hiding going on here, or rearranging things within the module: (almost) everything now has to be marked 'pub' to let other sub-modules in the new options module to see it.