Allow updating only part of the buffer

Thank you for this great driver! *Máš u mě pivo* :)

Here is a change that allows updating only part of the buffer at a time. I used this to make a simple game: https://www.youtube.com/watch?v=fwyFTVJoppA&index=20&list=PLFt-PM7J_H3GK-Ry_7X2pKdVfdlUPzZM2

The game was created with version 1.1 of micropython-ws2812, which was too slow to do a full update in each frame. From the speed point of view, it might not be necessary anymore. But it's still good for memory: I can "paint" objects onto the buffer, rather than preparing everything in an extra list and then `show()`ing that.