Hooks

- Fixed typo in example hook scripts.
Hooks

- Added README describing the hook system.
- Corrected hook interface details regarding exit codes.
Hooks

- Clarified the interface for on-exit, as there is no opportunity for scripts
  triggered by this event to modify data directly.
Hooks

- Removed code supporting data modification during the on-exit event.
- Fixed hook script interface descriptions.
Hooks

- Modified ::onExit to remove the indicator that data was modified. This is not
  supported.