Added autoUpdate functionality

When autoUpdate is enabled, the mask will be re-applied anytime the display list changes. It is off by default for backwards-compatibility. autoUpdate works by overridding methods such as addChild and removeChild. This means that if someone was to repeatedly call those methods (in a loop for example) there would be a performance impact.

I decided to add the autoUpdate param in the constructor. I think it makes sense but I'll let you decide. Cheers!