Feature: xml: allow specifying alertable events

This allows the user to specify that an alert agent should be called for only a
subset of events. With the following example, the agent would only be called for
resource events and changes in the "myattr" node attribute:

<alert id="my-alert" path="/path/to/my-script.sh">
  <select>
     <select_attributes>
       <attribute name="myattr" />
     </select_attributes>
     <select_resources />
  </select>
...