Previously plugin initialization happened in several steps:

 1. The plugin constructor was called with the root element and config
    options
 2. The plugin's `annotator` property was set to a reference to the
    `Guest` instance for the current document
 3. The plugin's `pluginInit` method was called

There was no real need for this multi-step process. This commit
simplifies it by removing the `pluginInit` method and passing the guest
reference as a third argument to the constructor in step 1.

As a result, the `Plugin` base class is no longer needed. Plugins now
extend the parent `Delegator` class instead.