Add support to read name of Pipfile from environment variable

[DonJayamanne]

Name of the Pipfile is currently hardcoded in the extension.
This could come from an environment variable (see here https://github.com/pypa/pipenv/blob/5f48233637adc99bb16e6c601a728dc641bd5cd2/pipenv/environments.py#L29)

TODO: Confirm this is the case and create necessary PR accordingly.

[brettcannon]

In this instance we may need to just use pipenv --venv or some such command to do the detection appropriately.

[MikhailArkhipov]

Yes, running command is a better test, but watch out for performance. This was the reason for the file check. Interpreter selection and startup time.

[DonJayamanne]

@brettcannon @MikhailArkhipov
Not sure what you two are referring to here. Running any pipenv command will not tell us the name of the Pipfile. The name of the Pip file can be provided in the environment variable:

# Specify a custom Pipfile location.
PIPENV_PIPFILE = os.environ.get('PIPENV_PIPFILE')

All we need to do is:

• Use the environment variable if it has been defined and the file exists
• Else we fall back to the default of Pipfile (current behavior)

[brettcannon]

What I'm saying is we simply don't care about a Pipfile and instead just care if pipenv knows of a virtual environment for the directory.