patch 8.1.0495: :filter only supports some commands

Problem:    :filter only supports some commands.
Solution:   Add :filter support for more commands. (Marcin Szamotulski,
            closes #2856)

src/option.c

@@ -10083,6 +10083,10 @@ showoptions(
 	item_count = 0;
 	for (p = &options[0]; p->fullname != NULL; p++)
 	{
+	    // apply :filter /pat/
+	    if (message_filtered((char_u *) p->fullname))
+		continue;
+
 	    varp = NULL;
 	    isterm = istermoption(p);
 	    if (opt_flags != 0)