Associate project dir with new virtual environment

This lets pew know which directory to switch to when activating an
environment with `pew workon foo`.

pipenv/core.py

@@ -904,6 +904,8 @@ def do_create_virtualenv(python=None, site_packages=False):
             'new',
             project.virtualenv_name,
             '-d',
+            '-a',
+            project.project_directory,
         ]
     # Default to using sys.executable, if Python wasn't provided.
     if not python: