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.c

@@ -8626,3 +8626,10 @@ cmd_paste(ProfWin *window, const char *const command, gchar **args)
 
     return TRUE;
 }
+
+gboolean
+cmd_color(ProfWin *window, const char *const command, gchar **args)
+{
+    _cmd_set_boolean_preference(args[0], command, "Consistent color generation for nicks", PREF_COLOR_NICK);
+    return TRUE;
+}