Add a cop that checks for initializer returns

[Drenmi]

I am seeing more and more code like this:

def initialize
  foo
  return :qux if bar?
  baz
end

I don't think it's very useful to return from an initializer, or at least I have never had a need to do so before. Even if there are cases where that would make sense, it is not possible to use a return value from an initializer.

I suggest a cop that marks returns from initializer methods as offenses or, alternatively (if someone can provide a strong rationale for still doing that) marks code that tries to return an explicit value from an initilaizer.

---

Expected behavior

RuboCop registers an offense.

Actual behavior

RuboCop doesn't register an offense.

Steps to reproduce the problem

Run bundle exec rubocop on:

def initialize
  foo
  return :qux if bar?
  baz
end

RuboCop version

Include the output of rubocop -V. Here's an example:

$ rubocop -V
0.47.1 (using Parser 2.3.3.1, running on ruby 2.3.3 x86_64-darwin15)

[mikegee]

This reminded me that the return value from setter methods is ignored. We could make a cop for that too.

pry> class A
pry*   def foo=(bar)
pry*     return 42
pry*   end
pry* end
pry> a = A.new
pry> a.foo = :some_value
:some_value

[harold-s]

ConfigurableEnforcedStyle uses a return in the detected_style= setter method.
Will fix my PR later to modify this file.