Add support for `getBuffer`/`getBufferAsync`

[aardvarkk]

It looks like these functions are brought it from Jimp's core, but a fairly simple .png encoding (using getBufferAsync) appears to take a few hundred milliseconds in my application and is becoming a pretty big bottleneck.

Sorry to just request features and not be implementing them, but thought I'd call this one out as one that's slow in Jimp-land as well.

[sjoerdthedev]

I'll look into it! Also no worries about the just requesting bit, just pointing out bottlenecks is already very helpful when it comes do deciding what should get optimized next.