Andrew flexible sign in

Instead of being stuck with this:

```python
py.sign_in('me', 'my_key')
```

you can now do:

```python
py.sign_in('me', 'my_key', plotly_domain='my plotly', plotly_streaming_domain='my streaming')
```

This came up for development and testing. We should be able to change these per-session. We *could* hide all of this from the user, but i don't see the harm in exposing it.