boolean() and overlayWith() accept raw buffers as input

[mhirsch]

This PR allows boolean() and overlayWith() to accept raw buffers, just as sharp() does. It uses the same options.raw syntax as sharp().

[lovell]

We'll need much higher test coverage at this complexity level. However a better approach here is probably to abstract the common logic for the various file/Buffer/raw input to a separate function and keep the boolean and overlayWith functions themselves simpler.

[coveralls]

Coverage decreased (-0.4%) to 98.654% when pulling bba141a on mhirsch:boolean_raw into 032bb7e on lovell:master.

[lovell]

Commit e6bfa52 on the pencil (v0.16.0) branch introduces quite a significant refactor to abstract the common image opening logic, which just so happens to allow the use of raw pixel data with any method that takes an input image.

I took the liberty of using the test cases from this PR to prove this is the case :)

I'll close this PR as it is no longer necessary but, most importantly, thank you for kicking me into action to clean up the code and make it more extensible for others to contribute.