Add DSL for the deletion

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
```ruby
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