Allow pipenv to respect an external configuration to change the default pip mirror

[ninrod]

@kennethreitz, I have a simple use case that directly if affected by #1040 and #856

I have a project that uses pipenv. I write this project both at home and at work.
At home, I have no problem, everything works. At work, I have to use the internal pipy index.

As the Pipfile is commited and pushed to git, I cannot keep changing it as I hop from home to work.
I would like to have an external configuration that signals to pip that I have to use another index.

putting this mirror config inside the Pipfile does not work.

So allow pipenv to use a global configuration, external to the project, to overwrite the default pipy index.

[kennethreitz]

closing as we have many existing issues that reflect this