commons.hub_state.py needs more

[littlebee]

There are several things I find lacking in the current implementation of basic_bot.commons.hub_state.

1. For example, there is no example of how to fetch an individual key. Probably because the accepted way is to just reach into the state member variable and do a .get on that like, hub_state.state.get('some-key'). See below for more about this.
2. There should be an overload of .get method on basic_bot.commons.HubState that accepts a singular key and does a .get on the state var to get and return a singular value
3. Doc comment for the get method need to explicitly swear allegience to the Pythonic way of .get method and what is returned / happens when keys don't exist in the state. The current method is behaving as I might expect - keys not found are not added to the return value. Singular version should just return None if key not found.