CustomEvent observer for render event (waitForJSEvent) #132
Description
Some applications may contain certain display logic that results in the page layout changing from an primary layout (i.e. portrait) to a secondary one (i.e. landscape).
Electron PDF should support this by allowing clients to register an observer for the CustomEvent.detail object when listening to the IPC channel from the browser.
The observer will be invoked before the page is captured
The API must also contain a new function to allow arguments set during construction to be updated with new values.