Allow SDK to automatically generate Auth from environment variable

[tbarsballe]

In the V1 SDK, if the environment variable PL_API_KEY contained a valid API key, it was automatically used by the SDK (and CLI) for authentication.

It would be nice if this behavior was retained in V2, such that I could instantiate a Session() without needing to first initialize and write() an Auth object if this environment variable was already defined

[sgillies]

@tbarsballe I agree, this is the way I want to work with the client while I'm developing. I've got a candidate solution in #589.