Refactor extra::term a bit

Move all the colors into a nested mod named color instead of prefixing
with "color_".

Define a new type color::Color, and make this a u16 instead of a u8 (to
allow for easy comparisons against num_colors, which is a u16).

Remove color_supported and replace it with num_colors.

Teach fg() and bg() to "dim" bright colors down to the normal intensity
if num_colors isn't high enough.

Remove unnecessary copies, and fix a bug where a terminfo parse failure
would try to use the wrong error and end up failing.