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 :-)