Support use on directories

[Zac-HD]

In all my uses of pyupgrade, I want to apply it to every Python file in the repo - in general, some directory. pyupgrade $(find . -name '*.py') works OK on *nix systems, but I'd be happier if I could just write pyupgrade . and have it work on Windows too!

Personally I'd prefer to follow Black and flake8 (etc) by having recursive-by-default for directories, but if not a -r/--recursive flag is also fairly common.

[asottile]

see #40 #46 asottile/add-trailing-comma#51 asottile/add-trailing-comma#59 asottile/reorder-python-imports#76