Add 'not implemented' errors for unimplemented flags

These used to live in TODO but we got rid off them.  Make sure
we keep track of what we want to support by printing a message
when those flags are unimplemented.

expand.c

@@ -14,7 +14,7 @@ static int expand(Fdescr *f, int tabstop);
 static void
 usage(void)
 {
-	eprintf("usage: %s [-t n] [file...]\n", argv0);
+	eprintf("usage: %s [-i] [-t n] [file...]\n", argv0);
 }
 
 int
@@ -25,6 +25,8 @@ main(int argc, char *argv[])
 	int tabstop = 8;
 
 	ARGBEGIN {
+	case 'i':
+		eprintf("not implemented\n");
 	case 't':
 		tabstop = estrtol(EARGF(usage()), 0);
 		break;