Update appy.js

Even though there is code in place to make the port selection of the application highly configurable, there needs to be a way to define the host to which the process binds to, in the same configurable manner.

For example when exposing to port :80 via nginx you don't want the site to be available on :3000 as well. This could be solved with a firewall rule, but then this may not be available to the developer deploying the application.