Add export function setup() {}

[gr2m]

I've ran several times into situations where I thought it would be great to have a setup script that is run only once, and before anything else. We can somewhat workaround it by moving all the logic into the script() function and make sure it only runs once, but the problem with that is that the script function is only run after all repositories are loaded, which can take quite a while if you have hundreds of them.

Use cases

1. Validate options
2. Load data from a template repository, e.g. see https://github.com/bdougie/octoherd-script-copy-labels