Added check to preserve ratio of the crop on scaling.

This is configurable and uses ratio option set while invoking darkroom

Original quote from @vpratti:

> Thank you for coming up with this plugin. It was very useful. I found
> a small bug in the crop plugin, which is while an crop is scaled and
> ratio is set, the crop does not respect the ration set once it hits
> either the boundaries of width or height. This is a fix for that.
Update maxHeight example