Simple syntax for batching migrations in `generate-script`

[dylan-smith]

We have inventory-report for ado2gh and soon to be in gei for powerful batching and sorting flexibility. But alot of customers don't need that much flexibility, and just want a simple way to say "group my migrations into batches of 500 repos".

We could add an arg to generate-script something like --batch-size that would output several migration scripts, with 500 repos in each.

Customer could also accomplish the same thing with inventory-report and splitting up the csv's. But this could be a more convenient option for customers that want to do simple batching.