Ensure last event within debounceDelay window is emitted

[bengourley]

I came up against the issue in #142 where the last change event is discarded. This ensures that the last event will fire after some are debounced.

I used the test that @motemen created in #142 but I found a simpler way to implement the logic.

In the grand schema of things I think debouncing/throttling should be left up to the user (re: your comments @shama #142 (comment)) but since this is already inside the module I thought I'd fix the bug. Feel free to remove everything later :)

[bengourley]

I added another commit to implement the debounceImmediate: true|false option that decides whether an event should be emitted on the leading edge of the window (terminology borrowed from https://npm.im/debounce). This means that the internal debounce function is feature complete and usable. I set the default to true which means the existing behaviour is maintained.

[bengourley]

Closing because of failing test on various platforms and lack of activity.