Allow user to stop the parsing process

[qcam]

This feature will be useful in case we want to stop parsing and quickly return data in the middle.

For example we are only interesting in the first <bar> tag in the underneath XML document.

<?xml version="1.0"
<foo>
  <bar bar="1st bar"/>
  <bar bar="2nd bar"/>
  <bar bar="3rd bar"/>
...
    <bar bar="nth bar"/>
</foo>

With this feature we can quit parsing and quickly return once we receive the <bar> element.

defmodule MyHandler do
  use Saxy.Handler

  def handle_event(:end_element, {"bar", attributes}, state) do
    {:stop, attributes}
  end

  def handle_event(_other_event, _, _) do
    # no need to write this actually :(
  end
end