Invalid arguments are silently ignored

This works:
```
wl-paste --watch cliphist -max-items 20 store
```

Both of these actually use `-max-items 750` (the default):
```
wl-paste --watch cliphist store -max-items 20
wl-paste --watch cliphist store --max-items 20
```

No error or warning is displayed. Usually, when CLIs receive invalid or unexpected input (expected no more arguments), it prints usage and quits.