A call-back which is executed during Velocity merge before a reference
value is inserted into the output stream. All registered
ReferenceInsertionEventHandlers are called in sequence. If no
ReferenceInsertionEventHandlers are are registered then reference value
is inserted into the output stream as is.
reference - Reference from template about to be inserted.
value - Value about to be inserted (after its toString()
method is called).
Object on which toString() should be called for