Add on attached listener to CountdownView to fix #45

[ahaverty]

Added an on attached interface listener to allow starting the countdown again with specified millisecond long to fix #45
Added in the event that onDetachedFromWindow is called (which stops the countdown)
onDetached occurs if list items in a recycler view go out of view

[ahaverty]

The idea is to start the countdownView and also set the listener in your onBindViewHolder of a recyclerView

For example:

 countdownView.start(eta);
 //Fix the out-of-view pause issue, tell it what time to use once it reattaches
 countdownView.setOnAttachedToWindowListener(() -> time - System.currentTimeMillis());

[ahaverty]

Confirming that it has fixed the pause issues in my own recycler view. Thanks for a great repo @iwgang 👌