Color output with ANSI codes

Colorize special tokens (#hashtags, @mentions, urls) with ANSI codes.
Introduce new configuration variable (colorize).
Check if output is a TTY to avoid redirecting/piping garbage.

Signed-off-by: L. Alberto Giménez <agimenez@sysvalve.es>

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I gave a shot to ncurses but its ending method clears out the screen and I didn't find a way to disable that behaviour (if it is possible at all). Also, implementation with ANSI color codes is simpler and not less standard.