Add 256 color support

[sebastinas]

This is required for bpython/bpython#193.

[jquast]

I've been studying 256 color support for some time. It is problematic, as there are two forms. The 24-bit nearest-match true color support form would be preferred, but is not supported by all 256-color capable terminal emulators, which may support only the 256 color palette.

https://gist.github.com/XVilka/8346728

In general, this is very hard to do universally. as the TERM type is not enough information to know which form of the 256 color support the client may support.

[thomasballinger]

See https://github.com/thomasballinger/curtsies/issues/80, once FmtStrs output is backed by Blessed colored strings this will make sense.