When the value of a configuration parameter is altered, either directly or by loading a configuration file, a configuration event is generated. Handlers can be registered to be notified when a configuration event occurs. A configuration event is associated with the group and key that had its value modified.
If a configuration event is emitted by a configuration context, it propagates to all configuration contexts which inherit that group and key from it.
A configuration event handler must be a closure that accepts three arguments:
handler cfg group key
cfg is always the configuration context that received the event, and the group and key identify the configuration parameter that changed.
Registers handler to be called when configuration changes in the context cfg.