hydra token user: add ability to not terminate on successful auth flow

[verzac]

Either way, this is a nice-to-have, so it's not 100% necessary - just wanted to get the idea out there and looked at just in case a contributor comes along and wants to contribute. Happy to help when I have the time though!

Is your feature request related to a problem? Please describe.

When I'm testing the auth flow locally with the custom IdP I'm developing, I'd like for the token client to not close when auth flow is complete, so that I can keep on testing live changes to the auth flow iteratively.

Describe the solution you'd like

Make this line here optional: https://github.com/ory/hydra/blob/master/cmd/token_user.go#L238

We can probably do this through a very optional flag called --no-shutdown.

Now, I know it generates a nonce and a state when it starts up, and I'm not 100% aware if nonces/states can be reused when auth-ing with Hydra (it probably can't and shouldn't), so we might need hydra token to reset when GET /callback is called.

Describe alternatives you've considered

It's just a nice-to-have for lazy bums like me, so the alternative is running hydra token user again and again every time I want to test the auth flow.

[aeneasr]

Sounds good to me! Would you be up for a contribution? :) state and nonce should probably be regenerated when we redo the flow :)