Bind to host address

[jirenius]

Issue

Resgate needs to allow binding to host IP address.

Solution

A new configuration option to be added:

-i, --addr <host>     Bind to HOST address (default: 0.0.0.0)

For json configuration file:

// Bind to HOST address.
{
    "addr": "0.0.0.0"
}

A null value or omitted configuration should default to "0.0.0.0".
An empty string ("") would bind to both ipv4 and ipv6.

[jirenius]

Solved in #77