Add initial support for XEP-0392

The last 3 commits added basic support.
Thanks @aaptel!

This commit adds basic settings interface to use it.
See `/color on|off`.

We still have to enable settings for color blindness.
And maybe another setting to decide whether to color the
occupantslist/roster with the same algo.

Regards https://github.com/profanity-im/profanity/issues/1191

src/command/cmd_funcs.h

@@ -223,4 +223,6 @@ gboolean cmd_save(ProfWin *window, const char *const command, gchar **args);
 gboolean cmd_reload(ProfWin *window, const char *const command, gchar **args);
 
 gboolean cmd_paste(ProfWin *window, const char *const command, gchar **args);
+
+gboolean cmd_color(ProfWin *window, const char *const command, gchar **args);
 #endif