Add a way to set run loop mode

Hello, thanks for creating this useful library. I love it much.

But I've noticed that GIF animation playback is deferred until scroll stops when used inside UITableView. The issue could be easily mitigated by changing CADisplayLink's run loop mode for `NSRunLoopCommonModes`, so I've added a way to pass run loop mode to `startGIF`.

Any feedback will be greatly appreciated!