add cop for "global method"

[grosser]

feature request

We have a project were many people contribute that don't know ruby well, they often write methods that are global, I'd like to stop them
this also often happens in things like Rakefile or *.rake files

Describe the solution you'd like

class Foo
  def not_global <- Good
  end
end
def this_is_global <- Bad
end

Describe alternatives you've considered

git grep '^def works, but would be nice if it's in rubocop

[bbatsov]

The idea makes sense to me, although such a cop should likely be disabled by default as for many scripts top-level method definition are perfectly fine.