Issue description

When the environment variable PIPENV_SITE_PACKAGES is set, pipenv always recreate new virtualenv on each invoke of pipenv even if it is not a pipenv install command.

Expected result

• pipenv run or pipenv shell should use existing virtualenv instead of creating a new one.
• PIPENV_SITE_PACKAGES should only affect pipenv install command.

Actual

mkvirtualenv --help
should print a help message and not create a virtualenv named --help. The same applies to almost all commands in compat_aliases.fish

i've got a bunch of Pipenv shell buffers and don't know which is which. It would be handy to name them when you create them, like I can do with ^U M-x shell. (I tend to name my shell buffers for the dir I'm in working in to help me remember).

Thanks for pipenv.el.