report_site from whatweb xml

This PR adds a basic importer for WhatWeb XML files. This initial iteration only performs a report_site from the data gathered. 

The next step is to add notes from the data acquired. Whats the best way to do this? Simply convert the XML elements in <target> to a hash and save it? Its the most complete but we would have redundant data from the site report and potentially a bunch of garbage we dont want. Alternately we can select specific plugin tags we want to rip like what you see with the IP and HTTServer tags in the site report. Suggestion?