pyenv uninstall prompt is unclear

[orn688]

When I remove a virtualenv created through pyenv-virtualenv, a confirmation prompt shows up but does not specify how to confirm or reject the removal. It might just be me, but my first instinct is to just hit Enter, which silently closes the prompt but actually has no effect on the virtualenv.

It would be nice to:

1. Add a [Y/n] at the end of the virtualenv removal prompt like apt install uses.
2. Reject invalid inputs (require that the user input y* or n* (case-insensitive).

I'm happy to submit a PR for this if it seems reasonable, as it should be a pretty simple change. Thanks for a great tool!

Sample output:

$ pyenv virtualenv 3.7.2 test-venv
Looking in links: /var/folders/hz/yv66z2011rv4_894gl1_v__80000gn/T/tmpa6sfk3lj
Requirement already satisfied: setuptools in /Users/onewman/.pyenv/versions/3.7.2/envs/test/lib/python3.7/site-packages (40.6.2)
Requirement already satisfied: pip in /Users/onewman/.pyenv/versions/3.7.2/envs/test/lib/python3.7/site-packages (18.1)
$ pyenv uninstall test-venv
pyenv-virtualenv: remove /Users/onewman/.pyenv/versions/3.7.2/envs/test? <Enter>
$ # no output, virtualenv still present