Support for callbacks to handle elements without adding them to the collection

This comes handy when we want to process huge files and we don't really
care about having them in the collection.

```ruby
class Product
  include SAXMachine
  element "ns2:name", :as => :name
end

class Products
  include SAXMachine
  elements :product, :as => :products, :class => Product

  def handle_product(product)
    product.is_a?(Product) # => true
    product.name # => asdf
  end
end

file = File.open('hugefile.xml')
parser = Products.new
parser.parse(file)
parser.products.length # => 0
```