Pyenv-virtualenv: Set the pipenv venv automatically

[bryant1410]

Apart from the fact that pipenv supports pyenv to install missing Python versions (#981), I think it should be cool if pyenv-virtualenv automatically calls pipenv shell if there's a Pipfile (or activates its environment).

The handling of the paths and the shell is not a responsibility for pipenv, so I think pyenv-virtualenv should do it.

After pyenv/pyenv#1020

[bryant1410]

As a workaround, one can create a symlink to the pipenv virtualenv like this:

ln -s ../../.local/share/virtualenvs/project-EN26V0ef ~/.pyenv/versions/project-EN26V0ef

And then just set it:

pyenv local project-EN26V0ef