new_address() fails for OfflineWallet
Why can't I generate random **new_address()** subaddresses for OfflineWallet?
For random address generation collision is less likely.

```
    return self._backend.new_address(account=self.index, label=label)
  File "C:\Python36\lib\site-packages\monero\backends\offline.py", line 49, in new_address
    raise WalletIsOffline()
monero.backends.offline.WalletIsOffline
```

Currently it requires open wallet file in RPC. 
Offline example uses get_address(major, minor)
https://monero-python.readthedocs.io/en/latest/backends.html