Add a first-class command line interface

[browniebroke]

Description

The current usage requires to setup libCST and invoke the Django Codemod commands from it.

It would be nicer if users could have a first class CLI specific to the use case of upgrading Django.

Examples of usages I've thoyght of, which I think would be nice:

# Run all codemods to jump from 2.2 to 3.1
django_codemod --from 2.2 [--to] 3.1

# Run all codemods to jump from the current installed version to 3.1
django_codemod [--to] 3.1

# Run just codemods for 3.1
django_codemod --only 3.1

# Run a specific modifier
django_codemod --mod django.shortcuts.render_to_response

The CLI should call libCST under the hood, without the end user having to worry about implementation detail.