Add a way to configure the binaries path

[sylbru]

Great idea, seems to work as expected. However I think an important missing feature would be to be able to specify ImageMagick binaries' path.
This wrapper is useful for hosting services that don't offer the Imagick PHP extension. In my case they offer to install ImageMagick binaries but place them in a folder like /opt/misc/imagemagick/bin, so I guess many others use many different paths instead.

[calcinai]

That makes sense - would you like to make a PR? I'm guessing it'd need to be done via a static var in the class - have a look at :: locateLinuxConvertBinary()