ChunkyPNG can now trim solid borders from images.

I suppose it's not a particularly useful feature given how long ChunkyPNG has been around, but every now and then one needs to trim a solid border of color from around an image, and this pull request provides that functionality. I had a nil check in at first, but that turned out to be unnecessary. This is my first contribution to an open-source project, so I hope it makes it in. :sweat_smile: