Be more specific with Nokogiri check

There are still strong reservations about using Nokogiri to parse
untrusted XML data.

http://www.wireharbor.com/hidden-security-risks-of-xml-parsing-xxe-attack/

It is also believed that many desktop operating systems are still
shipping out-of-date and vulnerable libxml2 libraries, which become
exposed via Nokogiri. For example:

http://stackoverflow.com/questions/18627075/nokogiri-1-6-0-still-pulls-in-wrong-version-of-libxml-on-os-x

While this isn't a problem for binary builds of Metasploit (Metasploit
Community, Express, or Pro) it can be a problem for development
versions or Kali's / Backtrack's version.

So, the compromise here is to allow for modules that don't directly
expose XML parsing. I can't say for sure that the various libxml2
vulnerabilities (current and future) aren't also exposed via
`Nokogiri::HTML` but I also can't come up with a reasonable demo.

Metasploit committers should still look at any module that relies on
Nokogiri very carefully, and suggest alternatives if there are any. But,
it's sometimes going to be required for complex HTML parsing.

tl;dr: Use REXML for XML parsing, and Nokogiri for HTML parsing if you
absolutely must.
Refactor check_nokogiri in msftidy
Land todb-r7#8, better nested if
Land #3405, improved Nokogiri check for msftidy