Plugin loading chain

The plugin initialize itself in a few steps:

1. Plugin loaded by WordPress (or theme/plugin if it's bundled)

2. On init 5 Notification plugin is initialized - all the classes and functions are loaded

3. do_action( 'notification/init' ) - Plugin is initialized. Safe for extending

4. do_action( 'notification/elements' ) - All defaults (Carriers, Recipients, Triggers, Merge Tags and Resolvers) are loaded. Extensions should register custom elements on this action