Accept a custom domain as a tunnel parameter

[peterwurzinger]

Hi,
First of all, thanks for your work! :)

Currently it's not possible to pass a custom domain to a created tunnel. When testing applications that need to process webhooks, those webhook endpoints are normally passed as static URIs.

It should not be that hard to implement that requirement, some quick tests according to the docs would indicate, that it's only a matter of passing a domain property in here. Since the API client already holds a options instance, it would make sense to extend those by string? Domain.
Since it's a optional value anyway, it wouldn't lead to a breaking change.

Would you accept a PR?

Cheers