Add setting to not colorize own nick according to xep-0392

Some users might want there nick to always stay white (etc) for easier
recognition.

Now we can do `/color own off` to not generate the color based on
xep-0392. The `me=` color (etc) from the theme will then be used.

Once we run this command `theme_load()` is called again.
And the theme looks totally wrong.
We encountered this at other times already and I think it's nothing
wrong with this new code here now but that there seems to be a missing
closing attr for the color when drawing.

Should be investigated seperately.

Fix https://github.com/profanity-im/profanity/issues/1288

src/command/cmd_funcs.c

@@ -8694,6 +8694,10 @@ cmd_color(ProfWin *window, const char *const command, gchar **args)
             prefs_set_string(PREF_COLOR_NICK, "redgreen");
     } else if (g_strcmp0(args[0], "blue") == 0) {
             prefs_set_string(PREF_COLOR_NICK, "blue");
+    } else if (g_strcmp0(args[0], "own") == 0) {
+        if (g_strcmp0(args[1], "on") == 0) {
+            _cmd_set_boolean_preference(args[1], command, "Color generation for own nick", PREF_COLOR_NICK_OWN);
+        }
     } else {
         cons_bad_cmd_usage(command);
         return TRUE;