Add repl module and entrypoint

[radrogow]

Also, UserContextFile, to work on live files

Closes #142

[BrianPugh]

Review: I think the code looks fantastic, thank you! While giving this a test, I instinctively wanted the following commands: {mkdir, mv, cat, cd}. What are your thoughts on adding these commands?

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General comments that are not specific to this PR, but I noticed while reviewing:

1. We should probably have UserContext be an abstract baseclass, and the current implementation should be something like UserContextInMemory. Currently the inheritance is kind of not correct. E.g. the existing UserContextWinDisk never calls any super(). But, it's probably best not risking breaking anything with a backward incompatible change for what is essentially an internal refactor.

[radrogow]

What are your thoughts on adding these commands?

Done. As well as some more :)

About the "general comments" - I agree, with all points.
For eventual changes I'd suggest protocol instead of abc

Could you please publish a new version to pip when merged?
I'd like to use this code, but don't like to change my requirements.txt to git

[BrianPugh]

this all looks good! i'll merge/release soon! Thank you!