Support resizing with crop

[NARKOZ]

Is there any way to use geometry 400x300# as in imagemagick?

It resizes and crops if necessary to maintain aspect ratio (centre gravity) http://markevans.github.io/dragonfly/imagemagick#processors

[did]

@tomasc, do you think we could rely on the ImageProcessing gem? https://github.com/janko/image_processing/blob/master/doc/vips.md#methods.
It looks like they provide all the transformations helpers we'd need for Dragonfly.
What are your thoughts about that?

[tomasc]

@did I think so! Feel free to submit a PR.

[did]

that was quick! thanks @tomasc! Can't promise anything. I'll give it a try.

[tomasc]

@did will be cool, thank you!

[evenreven]

This is the only thing keeping me on the dumpster fire that is imagemagick. I really wish I had the skills to help with implementing this.