Goal Zero: Create automated test that includes full code base of largest OpenSource isort users

[timothycrosley]

This ticket is part of an aspirational goal for the isort project to achieve zero regressions through the remaining life of the 5.0.0 release, without slowing down the release velocity.

To help achieve this, isort should create an integration test suite that runs the latest master of isort against its largest existing OpenSource installations and ensures it doesn't change the output when using the project's given configuration.

As a starting point:

• django
• plone
• pip
• pandas
  ...

It should be made as easy as possible to add additional repositories to this test suite, so that if major isort users do encounter regression in the future, we can offer to add their project to the test suite to ensure stability going forward.