allow flag for location to flake8

[jpmorris]

I'm trying to run flake8-diff in an IDE pointing to a venv dir (without the venv activated), but the IDE only allows running a single command (with arguments and location) and not specifying environment (or activating a venv). Because of this the 'which flake8' in the code dosn't find flake8. I've tried to install flake8 system-wide (not something I should have to do) and I'm stuck trying to find out trying to get flake8 to recognize my flakehell install and all my configs. This would be a lot easier if there was a simple flag that pointed to the flake8 location