add option to parse nmap results in grepable format

One of my other projects automates nmap scanning amongst other things and I have nmap output in grepable format which I wanted to be able to feed into Lair. This PR adds an optional parameter to the standard drone-nmap script to parse file/strings as grepable output from nmap. This doesn't impact standard drone-nmap usage. If nothing else, it was an exercise in parsing strings with regex!