Add printing external (internet) IP address

[philippgille]

Currently (v.0.2.1) only LAN IP addresses are printed, but in case the user set up port forwarding in his home network or uses serve on a VPS, the external IP address that's reachable from the internet would be useful as well.

Use 3rd party web services to get the external IP address:

• http://ifconfig.co/
• https://httpbin.org/#/Anything/get_anything (origin response)
• others

Make requests to multiple services in case 1) one goes offline in the future and 2) the responses differ (then use the address which most services responded with).

Two things need to be kept in mind:

• Make this optional (maybe true as default, but the user should be able to disable it), because some user's might not want any 3rd party programs to make requests to other services
• When using with -s, the external IP address should be included in the SAN entries