Option to restrict webserver to localhost

[passw0rd123]

This should fix #418

• Adds a localhostonly option to the config
• Ability to choose between listening on all interfaces or 127.0.0.1 (v4 only for now)
• Default is preserved for backward compatibility (listen on all interfaces)

Code is untested for now. So please have a look at the changes.

I chose this boolean option (all/localhost) to avoid the need of validating the config input as a correct ipv4/v6 address or empty string(->all interfaces). This might lead to more complexity and maybe dependencies which I like to avoid.