Allow users to dictate service enumeration scope

[stevecoward]

Currently enumerator will intelligently pick found services from the initial nmap scan and fire off further service enumeration per service discovered.

A pentest engagement may possibly be scoped to only identify and evaluate particular services (exclusively http/https, for example) so enumerator should not scan and enumerate other services. If a specific command line argument is present, enumerator will only register and enumerate those services specified. If this argument is not present at runtime, enumerator's default operation will be to enumerate all discovered services.