Eventually, there will probably be a `crate` visibility specifier that
does the same thing as `pub(crate)`. This commit replaces `pub` with
`pub(crate)`, so when `crate` is available we can easily switch to it.
Color logic is fairly complicated, so moved it into its own
module.
A `Color` object now encapsulates the --color setting, which
stream we are printing to, and what color we are painting.
This way, Color::paint can just do the right thing when asked to
paint text.
Also added tests to make sure that --list and --highlight colors
are using the correct color codes.
