Add switch to suppress empty groups stdout

```
Need option to suppress stdout for empty groups. Why?  pentesters+auditors 
might not care on existing servers (not exploitable). However, if auditing a 
base-build where future group members aren't clear yet, the auditor WILL care. 
Hence we need group-write for empty groups to be suppressable.
```

Original issue reported on code.google.com by `bernardo.damele` on 22 Oct 2012 at 9:07
