Wishlist. Added monitor selection and bounding box support. Added small test of each feature in the _demo section.  

As per your wishlist, I added in support for taking a snapshot of a single monitor. It enumerates all the available devices, and allows the user to select which they would like. 

Additionally, I added a boundingBox param while I was in there. In case people just need a small portion of the screen. 

_demo has a small test loop for verifying `saveScreenToBitmap` and `getScreenAsImage` work as intended.