Add a filter API to plugins to decide whether to track an event or not

[matus-tomlein]

Add an option to provide a filter callback in plugins that would return true or false deciding whether the event should be tracked or not. In case any plugin returns false, the event will be discarded and not tracked.

This will, for instance, enable filtering automatically tracked events such as from screen view autotracking and enable users to decide which ones to track (see issue #764)

The same issue on the Android tracker.