az repos delete to support --name

I'm splitting up a mono repo so am creating/destroying lots of repositories whilst getting the structure correct. `az repos create` works against the name, org and project, but `az repos delete` requires the id rather than the name.

Can we have an additional parameter that takes the repository name and internally does an `az repos show`.

I know I can write a wrapper to do this, which is what I've done, but having it natively would simplify matters and make it easier to hack from the console.