ColourTranslator rework;

Now supporting 'named colours', i.e. :red, :blue, :magenta, :yellow for 3-bit (0-7) terminals. Also supporting 'numbered colours' (0-255) for 8-bit terminals. Autodetects CSS style colours for both 8 and 24 bit colour modes.

Pass a Symbol: treated at 3-bit. Pass a Fixnum: treated at 8-bit. Pass a String: treated as 24-bit; if the terminal supports it.

Does not yet auto-detect Terminal support for 24-bit with graceful degradation to 8-bit.

Also, add more API documentation.