Added crop by primary point of interest given in percent/ratio

See #https://github.com/humanmade/WPThumb/pull/115

Crop from position works well, but sometimes you need to set a primary point of interest to base your crop on.

I've added just a few lines to crop_from_position() to allow for setting a ratio. '0.5,0.5' would be the same as 'center,center' but now you can also do something like 0.75,0,30.