Add an option with a color type.

Add an option with a color type.

Colors can be specified as "none" for terminal's default color, as a name of
one of the eight standard colors of ISO/IEC 6429 "black", "red", "green", etc.,
as an RGB hexadecimal tripplet #RRGGBB for 24-bit TrueColor, or as an
integer from 0 to 255.  Integers 0 to 7 are the synonyms for the standard
colors.  Integers 8-15 are used for the so-called bright colors from the
aixterm extended 16-color palette.  Integers 16-255 are the indexes into xterm
extended 256-color palette (usually 6x6x6 cube plus gray ramp).  In
general, 256-color palette is terminal dependent and sometimes can be
changed with OSC 4 sequences, e.g. "\033]4;1;rgb:00/FF/00\033\\".

It is the responsibility of the user to verify that the terminal supports
the specified colors.

PATCH v5 changes: documentation fixed.
PATCH v6 changes: documentation fixed.
PATCH v7 changes: rebase onto master and fixes after review.
PATCH v8 changes: fixes after review.