Generate namespaced interactors/organizers in the right directory

[Fixes #5]

rails g interactor foo/bar should generate app/interactors/foo/bar.rb

currently this generates

```ruby
class Foo::Bar
  include Interactor

  def call
  end
end
```
which is less than ideal, but follows how Rails does it
Yeah it looks like junk in the test but it also makes it clear exactly what the expected output is without concern over indentation. Active Support gives you [`String#strip_heredoc`](http://apidock.com/rails/String/strip_heredoc) but it's not available to us here. I'm comfortable leaving this as is.