Make download and exit link detection an Adobe Launch core extension event rather than part of appMeasurement.js

The auto detection of download and exit links provided by the Adobe Analytics appMeasurement.js is useful.  However, it would make far more sense to me if these were events provided by the core extension.

The benefit would be we would be able to:

• make the way these events are handled consistent with everything else we are doing (i.e. managing what happens through Launch rules rather than hacking stuff together using doPlugins)
• make these events available as a trigger event for other capabilities implemented through Adobe Launch (i.e. rather than just being an Adobe Analytics thing)