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").

src/plugins/exec/exec-command.h

@@ -33,6 +33,7 @@ struct t_exec_cmd_options
     int new_buffer;                    /* output in a new buffer            */
     int switch_to_buffer;              /* switch to the output buffer       */
     int line_numbers;                  /* 1 to display line numbers         */
+    int color;                         /* what to do with ANSI colors       */
     int display_rc;                    /* 1 to display return code          */
     const char *ptr_command_name;      /* name of command                   */
 };