Exclude ctop from the ctop list

This is a feature request... 

The point is, if ctop is installed, will be listed by ctop.

As far as is in constant usage, it could add noise and it applies to other services... so would be nice to add a feature to exclude containers (ie: ctop) from the list with a label like:

```
  --label exclude=ctop,netstat \
```

Thanks for the great tool!