Interface for classes which can dynamically load custom plugin rules associated with a user's plugin class.
Each plugin declaration has an associated RuleLoader instance, and that instance's addRules method is invoked each time the input xml specifies that an instance of that plugged-in class is to be created.
This is an abstract class rather than an interface in order to make it possible to enhance this class in future without breaking binary compatibility; it is possible to add methods to an abstract class, but not to an interface.
@since 1.6