Introduce a cropping process.
* Implemented resize_to_height.
* Reworked the way we pass loading options to the image reader. If you want
to enable sequential reading you have to explicitly call `sequential` before
manipulating it. That will set @_load_opts[:sequential] = true for jpegs and
pngs. More loading options (like shrink on load) could be added afterwards.
* Simplified the `@_vimage` declaration in `manipulate!`.
Bumped version to 0.2.0