pipenv open custom command help #3081

[metakermit]

As discussed in #3081 clarifies how custom commands can be used with pipenv open.

To test:

$ pipenv run python -m pipenv open --help
Usage: __main__.py open [OPTIONS] MODULE

  View a given module in your editor, as defined by the EDITOR environment
  variable. You can temporarily override it, for example:

      EDITOR=atom pipenv open requests

Options:
  --python TEXT       Specify which version of Python virtualenv should use.
  --three / --two     Use Python 3/2 when creating virtualenv.
  --clear             Clears caches (pipenv, pip, and pip-tools).
  -v, --verbose       Verbose mode.
  --pypi-mirror TEXT  Specify a PyPI mirror.
  -h, --help          Show this message and exit.