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.

The configuration file as committed basically will warn on anything that doesn't meet the suggested style guidelines in use by the Metasploit project, which are identical to Google's with the exceptions that:

* Lines are going to be a little longer and this is normal and just easier
* Methods, in particular the "main" methods called in modules like run_host, etc, are going to be long.  Perhaps 30 is way too low.
* Classes under modules are expected to be huge.  This is OK.

Unless this just dies right here, I see this configuration file changing over time and people using it more as a way to correct obvious style/etc issues but not (yet) necessarily making it a requirement to land.

The usage is simple.  With this PR:

# bundle install
# rubocop modules/some/module/here
# Deal with the problems