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
`<<-EOF.sub(/^\s{10}/, "")` (or whatever the right number of spaces is) and then indent the code below so it fits better.