add support for can_change_color

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Right now we only support the standard terminal colors. This works well enough, but it would be nice to get some other highlight colors, for example, blue is a terrible highlight color right now.

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Setting color values should be done through commands so a config script can set things up.

Note to self: set TERM to xterm-256color to get it working under gnome-terminal

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It looks like the OS X Terminal.app doesn't actually support changing colors even though it reports as much. The default color settings it has are probably good enough, so we can just initialize other terminals to use the same color set.

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Closing since we support 256 color terms now. Changing colors is probably not worth it right now since we can't change colors in all terminals.