Add external IP discovery module

This module performs an HTTP request to ifconfig.me/ip.
The body of the response contains the publicly routable IP from
which the request originated. This can be useful in discovering
routes on pivoted hosts and initial recon as a simple aux module.
msftidy and module placement per todb
Apply Egypt's cleanup

Remove revision, raise the exception itself, remove scanner mixin,
datastore['RHOST'] unstead of RHOSTS, and useles agent var removed.
Merge branch 'external_ip_discovery' of https://github.com/sempervictus/metasploit-framework into sempervictus-external_ip_discovery
cleanup before merge external_ip
use of send_request_cgi