add support for debouncing leading event

[ahamid]

debounceDelay only applies to subsequent events, not the initial event, which is always immediately fired. In the case that you want to wait for completion of initial activity on a file (e.g. sequential auto-saves in an editor), it's beneficial to debounce the leading event. This patch introduces debounceLeading which does just that - the leading event will not be fired immediately, but at the end of the debounce timeout.

Related issue is: #142

Debouncing the leading event (essentially forcing trailing event emission) coincidentally solve the above problem as well, however there is probably a more general way to solve both issues (see Underscore semantics of debounce and throttle with leading/trailing).

This patch has not been tested on master, but identical patch passes (existing) tests on 0.5.1.

[shama]

I wish how events got debounced/throttled was handled by the user as I don't want to rebuild those existing libs. But I can't think of a better way to handle it atm so merging this option. Thanks!

[tremby]

Please provide some documentation for this.