Add option to specify the crop gravity

[idflood]

By default the image is cropped in the center, but on some occasion it needs top be cropped differently. For instance a page screenshot is sometimes more usefull if we only keep the top of the page and cut the bottom.

ImageMagick has an option 'gravity' which seems to do exactly that.

[normanrz]

I added the option to the source code. Need to wait for npm release.

[boennemann]

@normanrz Before we publish, can you fix the build please? (:

And thanks for shipping this fast!

[normanrz]

Sorry, missed that. Fixed in e11da89

[idflood]

Wow, thanks for the quick addition, it works great 👍

[boennemann]

@normanrz Seem like the build is still broken: https://travis-ci.org/excellenteasy/grunt-image-resize/jobs/19970045#L361

[normanrz]

To make the tests work again, we need to wait for a bugfix in the underlying gm module (aheckmann/gm#260).