Circleflow snap

The existing behaviour moves the 'active' indicator smoothly between pages. I think it looks a little neater in some cases if the circle just appears at the new page instead, so I've added that behaviour as an option. The "snap" name comes from Jake Wharton's ViewPagerIndicator, IIRC, but the code is unrelated. With 'snap' turned on, it looks like the inactive indicator changes to the active indicator instead of the active indicator moving along.

I've also updated the README with a description of the new option.