Allow custom host name in registration

[maxfrigge]

Not sure if this is compliant or not, but it would be cool to set a custom host name when registering a service. This would allow creating a server, which can be found under a predefined domain - e.g. through a web browser.

Right now it looks like this case will always be true, because you cannot actually set HostName. It will thus fallback to os.Hostname() and thus depends on the environment. I have tested it using other values and it resolves and works fine under macOS.

I would be more than happy to create a PR, if you don't mind.

This would probably also help with the use case in #42.

[maxfrigge]

I have just found that this is what RegisterProxy is for.

Sorry for the noise!