Consider adding pre-commit hook for add-trailing-comma

[gtback]

https://github.com/asottile/add-trailing-comma

In the default configuration, this seems to conflict with our isort configuration (they try to overwrite each other). This will take some work to resolve. We should probably wait until we merge in the STIX 2.1 branch, to avoid having to make the same changes in multiple places.