Improve NSE scripts API and support

[savon-noir]

Today accessing NSE scripts output is possible via NmapService.scripts_results. This should be more documented and possibly provide a better API to access data. On top of this the following scripts output are not supported:

• hostscripts
• prescripts
• postscripts

Only service scripts are supported (most of scripts output).

[ifeatu]

+1 I'd like to run a script such as ssl-enum-ciphers (https://nmap.org/nsedoc/scripts/ssl-enum-ciphers.html)

Is there support for this today? Please advise!