Pyupgrade can now support directories as input

[saroad2]

Pyupgrade should be able to handle directories as inputs, like any other static code analysis tools (pylint, flake8, mypy, black, etc.)

Added this ability 😃.

[MarcoGorelli]

This looks like a duplicate of #150 #46 #339

[saroad2]

Hey @MarcoGorelli ,
I figured out that even though a lot of people have opened issues about recursive Pyupgrade, nobody actually did a PR to implement it. I thought that if I "just do it", than maybe the maintainers of Pyupgrade will consider merging it.
It does not damage any of the current functionalities, just adding a new one.

Moreover, I should state that the only reason I don't integrate Pyupgrade with my own projects, especially Statue, is because of the absence of this ability. I hope that now that I've shown that this is possible, you'd reconsider.