Add /occupants color command

`/occupants color on|off` to enable or disable XEP-0392 also for the
MUC occupants.

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

src/command/cmd_funcs.c

@@ -4442,6 +4442,12 @@ cmd_occupants(ProfWin *window, const char *const command, gchar **args)
         return TRUE;
     }
 
+    if (g_strcmp0(args[0], "color") == 0) {
+        _cmd_set_boolean_preference(args[1], command, "Occupants consistent colors", PREF_OCCUPANTS_COLOR_NICK);
+        occupantswin_occupants_all();
+        return TRUE;
+    }
+
     if (g_strcmp0(args[0], "default") == 0) {
         if (g_strcmp0(args[1], "show") == 0) {
             if (g_strcmp0(args[2], "jid") == 0) {