Prompt for credentials if not provided

[logan-pugh]

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Hi there, great work on this library, it is working very well for me so far.

One nicety I liked coming from sharepy is that it automatically prompts you for your username and password if you don't provide them. This makes it really easy to get started and use interactively (without worrying about shoulder surfers).

Any thoughts on adding that to shareplum? Thanks for your time and effort on this project!