Facilitate merging upstream changes into generated projects

[micahjsmith]

Add a command that, when run in a generated project repo, renders the most up-to-date project template from upstream DAI-Lab/cookiecutter-pypackage, and merges it into the current project.

See also:

• Add command to merge upstream updates to project template ballet/ballet#22
• Migrate template changes cookiecutter/cookiecutter#1134

[micahjsmith]

We think that this can be done robustly following ballet/ballet#40
I will work on a proposed implementation which we can review in a PR for discussion
@csala will also investigate whether he can come up with a much simpler solution that is equally robust