Add option to configure host

[magicmark]

We'd like to be able to configure the host that express binds to.
This new config option passes it along, and defaults to 0.0.0.0.

[ljharb]

Is 0.0.0.0 the current default?

[magicmark]

@ljharb yep!

Express uses http.Server.listen() (https://expressjs.com/en/api.html#app.listen)
Which defaults to 0.0.0.0 (https://nodejs.org/api/http.html#http_server_listen_port_hostname_backlog_callback)

[ljharb]

Instead of hardcoding the default, maybe it'd be better to have a default of null, and use the 2-arg form when a truthy host isn't provided?

[magicmark]

@ljharb Yeah sounds better, I'll do that.

Couple of things I wanted to ask you:

1. I wasn't going to typecheck config.host - logic being that http.server.listen will do its own typechecking, and an invalid host value error will bubble up for us. Does that sound reasonable? Or should we be going our own typechecking here?
2. In adding a test that the server doesn't blow up when specifying a host, I ran into an issue whereby I can't start a new instance of hypernova, because there's one already running from the previous test. Since the listen() and close() stuff is async, it requires a bit of refactoring to get into it to properly pass the done() callback (possibly exposing the 'server' attribute', or just adding a 'close' method to hypernova or something.) I can do this as a separate branch - any thoughts here?

Thanks!

[ljharb]

I prefer doing our own typechecking if possible.

If there's an issue with async tests, a separate PR to fix it, merged first, would be ideal.

[ljharb]

@magicmark would you mind rebasing on the command line, instead of clicking "update branch" on the github ui?

[magicmark]

@ljharb sure. I haven't touched this branch in a while though.

(waiting to finish another branch to refactor the integration test so I can write a test for this change)

[goatslacker]

would you mind rebasing on the command line, instead of clicking "update branch" on the github ui?

@ljharb that was me, my bad!