Simple Override Of DynamicCommand

This helps me to override DynamicCommand without having to do `Thor::DynamicCommand.class_eval`.

What I do now is

```ruby
class MyThorApp < Thor::App
  def self.dynamic_command_class
    DynamicCommand
  end

  class DynamicCommand < Thor::DynamicCommand
    def run(*args)
      # do my stuff here
    end
  end
end
```

I didn't add a test, yet, since I didn't know if this is the intended way of handling dynamic commands in Thor. Let me know when I'm wrong here :-)