Adds support for the --pypi-mirror command line parameter and the
PIPENV_PYPI_MIRROR environment variable for most pipenv operations.
This permits pipenv to function without pypi.org, which is necessary for
users:
1. behind restrictive networks
2. facing strict artifact sourcing policies
3. experiencing poor performance connecting to pypi.org
4. who've configured a local cache for performance reasons
When specified, the value of this parameter replaces all instances of
pypi.org and pypi.python.org within pipenv operations without modifying
or requring the modification of Pipfiles.
- Resolves#2075
Jacob Henner