Allow for customization of starting point of animation within an individual progress bar

[troutslaps]

Allows for passing a second argument to StoriesProgressView.startStories(...) which will allow the caller to specify at which duration to start the animation within the current progress bar from.

Example:

storiesProgressView = (StoriesProgressView) findViewById(R.id.stories);
storiesProgressView.setStoriesCount(6);
storiesProgressView.setStoryDuration(3000L);
// ...
// This will start the animation on half of the third progress bar
storiesProgressView.startStories(2, 1500L);```