Changed hostname default of localhost to null

Hi guys,

The default `localhost` binding can be bothering when you want to access your server from other devices for example.
You have to explicitly set the hostname value to `null` in the config.

By defaulting the value to `null`, the server can be accessed by any suitable pointer to it (IP, os hostname, localhost).
If the user wants to restrict the access, then the config can be modified to explicitly listen on a given hostname, `localhost` for example.