Add Rubocop to the Metasploit project

This is a work in progress because the code base is old and the style
varies wildly, however .rubocop.yml can (and should) be tweaked over
time to change standards as we see fit.  As it stands right now there
are few (if any) modules that pass Rubocop.
Add more sensible Rubocop defaults for line length.  disable top-level class documentation requirement for modules and don't require big numbers like 12737341238 to be separated with _ -- who does that?
Add Rubocop to msftidy.  You now have 15 seconds to comply. You are in direct violation of Penal Code 1.13, Section 9.
Rubocop checks are optional and info only

I like the change but it means that basically everything will fail
forever until we tweak up the config.
Just skip if info is suppressed.
Merge pull request #2 from todb-r7/rubocop-is-optional

Rubocop checks are optional and info only
Land #3528, add Rubocop from @jhart-r7

This adds the gem to the :development group in the Gemfile, as well as
wires up msftidy to use it.