Add an "immediate" argument

[nbish11]

This parameter can be useful in a lot of situations, for example:

// Using MooTools here
// A simple user-activity script

document.body.addEvent('mousemove', function () {
    this.set('data-useractivity', true);
}.debounce(250, true));

document.body.addEvent('mousemove', function () {
    this.set('data-useractivity', false);
}.debounce(250, false));

[m-gagne]

This PR would change the order of parameters for debounce, also if we pull this in, it should also apply to the limit function as well (I'm hoping to keep functional parity between the two).

[nbish11]

Hmmm, I see what you mean. While I could implement the immediate functionality to throttle(), it wouldn't really make sense if the immediate was required after context. Maybe it can be pulled into a future version?