add an image crop utility function

[stephentu]

Here's a simple function to crop an image into a bounding box. I needed this for feature augmentation, and figured it was useful independently for keystone.

[shivaram]

@etrain Should we make a new node for this ?

[etrain]

My thoughts exactly - a "Cropper"? it's implementation can be to call into this function directly.

[etrain]

I'm going to go ahead and merge this. I've created #211 to track creation of a Cropper but I think this is useful enough on its own.

[etrain]

Thanks @stephentu!