cmdutils: add -colors option

cmdutils.h

@@ -488,6 +488,12 @@ int show_layouts(void *optctx, const char *opt, const char *arg);
  */
 int show_sample_fmts(void *optctx, const char *opt, const char *arg);
 
+/**
+ * Print a listing containing all the color names and values recognized
+ * by the program.
+ */
+void show_colors(void *optctx, const char *opt, const char *arg);
+
 /**
  * Return a positive value if a line read from standard input
  * starts with [yY], otherwise return 0.