Allow virtual env creation in dir with leading dash

Invoke pew with a double dash separator ("--"), to make it clear that
the virtualenv name is a positional argument. Since the virtualenv
name includes all or at least the beginning of the directory name,
trying to create a virtualenv in a directory with a leading dash in
its name will cause pew to erroneously parse the virtualenv name as an
optional argument. Adding the separator causes the virtualenv name to
be parsed correctly.

Fixes #439

pipenv/core.py

@@ -914,7 +914,6 @@ def do_create_virtualenv(python=None, site_packages=False):
             '-m',
             'pipenv.pew',
             'new',
-            project.virtualenv_name,
             '-d',
             '-a',
             project.project_directory,
@@ -932,6 +931,8 @@ def do_create_virtualenv(python=None, site_packages=False):
         err=True,
     )
     cmd = cmd + ['-p', python]
+    if not project.is_venv_in_project():
+        cmd = cmd + ['--', project.virtualenv_name]
     # Actually create the virtualenv.
     with spinner():
         try: