Add the option to delete unused virtual environments

[bauca]

Hi Pythonist,
I noticed that after a while one might end up with many virtual environments stalling in the poetry's cache, where most of them won't be used anymore (e.g: you create a poetry virtual environment within a folder, and afterwards delete this folder). The previously virtual environment will be stalled.
So, the discussion here is: if I create a PR to for instance add a command to list all the virtual environments (regardless if I'm inside of a project), does it make sense?

poetry env list-all

the command above would list all virtual environments lying under the virtualenvs.path path.

I'd be glad to work on this feature, which for me makes a lot of sense, much better them keeping inspecting your