core: add functions to convert RGB to terminal color (and vice versa)
core: add options "term2rgb" and "rgb2term" in command /color
core: add modifier and infos to decode ANSI colors

New modifier:
- color_decode_ansi: convert ANSI colors to WeeChat colors (or remove colors).

New infos:
- color_ansi_regex: regex used to parse ANSI colors in a string
- color_term2rgb: convert a terminal color (0-255) to RGB
- color_rgb2term: convert a RGB color to terminal color (0-255)
irc: add modifier "irc_color_decode_ansi"
exec: add option "-color" in command /exec (decode ANSI colors by default)

The ANSI colors are decoded by default to WeeChat colors (for local display),
or IRC colors (if output is sent to buffer with "-o").