Adds support for bufferSize/numBuffer fields

These fields won't have any affect without my current branch of
node-speaker (hopefully to be merged into the main package), but cause
no harm on current versions.

These fields allow the application developer to tweak the balance
between audio delay and audio reliability.  Current buffers are far too
large for video-games on Windows, and cause a multi-second delay between
trying to play a sound and the sound actually playing, and these
settings allow the user to change that.