Extra arguments should trigger an error

```
% ./sping -peers 193.70.85.11 -debug.showstats foobar
2021/03/24 13:13:00 Listening on[::]:9523
it is now: 2021-03-24 13:13:01.000243893 +0100 CET m=+0.982975288
```

The extra `foobar` has been silently ignored, which is bad, specially giving the lack of documentation (see #4 )