Optionally remove virtualenvs when uninstalling poetry

[AdamWRichardson]

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the FAQ and general documentation and believe that my question is not already covered.

Feature Request

I'd like to add an option to the poetry installer which can remove the virtualenv directory as part of the uninstall. This could default to False to keep with current behaviour.

I'm happy to submit this myself if others would think it's a useful feature but I don't know where the installer script lives!