Provide a way to ignore warnings

Currently, with the newer 1.6 libpng, it generates warning when we have incorrect RGB profiles. However, the warnings are interpreted as errors by node-imagemagick-native.

Here is an example of such warning:  image.read failed with error: Magick: iCCP: known incorrect sRGB profile `' @ warning/png.c/MagickPNGWarningHandler/1830

This provides a way to silently ignore warnings (unless debug is on).