Should be implemented for custom code functionality inside a
Template. Implementing classes must have
a no-arg constructor. It is guaranteed that only one instance
of this class will be instantiated by the engine, although
multiple threads may possible use this class at the same time.
Called back from the template engine while substiting data inside
writer - can be used to write data at the point in the merged
file where the custom code was called. This writer
should not be closed, otherwise the template engine
will not be able to proceed after this method is invoked (since
the template engine also uses the same writer.
env - a reference to the template engine, can be used to
invoke methods in the engine.