deploy: add pre-deploy actions to TOML

[virtuald]

The idea is provide a way for the user to run a command

[[tool.robotpy.deploy]]
command = ["python", "-m", "mypy"]
required = False # if false won't stop the deploy if it fails

Allows users to do robotpy/pyfrc#206 if they want.

Arguably, this really isn't necessary -- users can already implement whatever they want by adding a 'deploy.sh' shell script and making everyone use that, but maybe this is better?

[auscompgeek]

Teams could also enforce this sort of thing via pytest today. Although I can see how that may be slightly awkward.