Add option to crop after scaling

[z38]

This PR adds a new option -r|--crop to crop the image after scaling. To implement this feature a new API call epeg_crop_set(Epeg_Image *im, int top, int bottom, int left, int right).

With this option, epeg can create thumbnails which are always of the same size (e.g. to be used for photo galleries).

In addition I renamed --inset to --outbound as most libraries tend to name the option like this.

Feel free to close this PR if this feature is not needed for most users or the implementation isn't well-coded.