Pipenv: Feature request: Cleanup unused venvs

I'm strongly in favor of the virtual environments being created in the same directory as the project for this reason. There's less "magic" and it's trivial to delete a whole project including its virtual environment.

So to be clear @jacebrowning, the functionality you're describing is still readily available with thePIPENV_VENV_IN_PROJECT environment variable.

@xi I think we could introduce this kind of functionality, but I'm not sure @kennethreitz is looking to add new commands to the API. Previously, when the virtualenv directory was located with the project by default, the user could use rm to delete the directory and start fresh. I think the pew commands are somewhat analagous to that, but it does require knowledge of some of the inner workings of pipenv.

At the very least, this should be documented, but perhaps Kenneth will have some thoughts on how to he'd like to see this best addressed.

@nateprewitt Thanks! I haven't actually read through the docs since they went up.