Add onEnd and triggerEnd options to trigger an event when the counter ends

[CristianCantoro]

As per the commit title, I have added two options:

• onEnd: expects a function that is triggered when the counter reaches the end. The default does nothing. A typical use is hiding the counter and showing a different - previously invisible - element of the page. This gives the effect of something appearing when the countdown ends. Here's an example:

$('#counter').countdownCube( {
  target: '2018-12-03T11:00:00Z',
  onEnd: function(e) {
           $('#counter').hide();
           $('#after').show();
         },
  } );

• triggerEnd: the default is false, if set to true triggers the onEnd function even after the countdown has expired. Otherwise, the counter shows the word LOADING....

[CristianCantoro]

I have also fixed an inconsistency in the behavior of the counter: when it reached zero it actually stopped at 00:00:01 (1 second) and did nothing if onEnd was not set. On the other end if the page was loaded and now > target (i.e. target is in the past), then the word LOADING... was displayed.

Now, in both cases, the counter is set to 0 when now > target. if onEnd and triggerEnd are set, the counter behaves accordingly.