added getFrameDelay method to read the animation delay of a given frame

Tested this with different gifs with short and long delays and also unevenly distributed delays.

It's the first emscripten code I've written so maybe there's an easier way to read and combine the second and third byte into an unsigned short. I couldn't find any api documentation for `getValue` etc.

Also this code was inspired by https://gist.github.com/keefo/78a083c148b9da2d2a40