Option to show diffs

[jayvdb]

Is your feature request related to a problem? Please describe.
When running djcodemod in a build worker, it would be useful to show the changes it made so the logs record the transformations made, and also so it is easy to copy the diffs to submit upstream.

This is especially useful when working from tarballs, as there is no VCS to be able to generate diffs.

Describe the solution you'd like
A disabled-by-default flag like --show-diffs which emits the changes which are being written to disk as patch-compatible stdout .

Additional context
e.g. it takes a couple of secs to add
`djcodemod run --removed-in 4.0 test_project/
to the build scripts for https://build.opensuse.org/package/show/home:jayvdb:branches:devel:languages:python:django/python-django-debreach , to allow me to provide a more detailed issue at lpomfrey/django-debreach#21

But I really should check the diffs to make sure djcodemod is not introducing bugs in the process, and if there were diffs I could have easily added to the upstream issue an inline patch for the first which djcodemod knows how to fix, which gets them closer to the harder second problem which djcodemod doesnt know how to solve.

[browniebroke]

PR welcome 👍