Expand Client API for Update and Similar Operations

[ZLLentz]

Expected Behavior

Two additions to the Client API:

• Client.update(dict) that performs the operations used in happi update <json> #207, but on the Client class. Accompany with a refactor to have the CLI arg simply call the new method.
• add exist_ok=False argument to Client.add_device so features like happi update <json> #207 can entirely use the public API

Current Behavior

No such API

Possible Solution

Add them, or something equivalently helpful after some discussion

Context

#207 needed to use the private API to accomplish its goals