Add DSL for the deletion

[kanevk]

Add DSL feature for the deleting service.

Currently the deletions are described through the Dalek::Exterminate.deletion_tree method via data structure.

It will be nice to have DSL build over the deletion tree structure.

Example

class DeleteUser
  include Dalek::Exterminate
  
  delete User do
    before { ... }
    
    delete :posts do
      delete :comments

      after { ... }
    end

    delete :comments do
      skip :images
      delete :tags
    end
    
    handle(:avatars) { ... }
  end
end