Add flag to login to generate non-persisted keys

Use Case: you are generating keys to put on a machine where you can't log in and run a web browser to do the auth0 flow. Instead, run `ironhide` on a machine where you do have a browser, and might already have keys generated, but run a `genkeys` command that causes IronHide to ignore the existing keys, generate a new set of keys, and just print the JSON to stdout. You can cut and paste it, redirect to a file, or pipe into an SSH that cats the input into a file on the remote machine.