crop images

	Utility for autocropping images. [crop body.ext] crops the border
	of the image [body.ext] and save the result in [body.crop.ext].


	For example,

		+------------------+
		|                  |
		|    /\ An image   |
		|   /  \----+      |
		|  /    \   |      |
		|                  |
		+------------------+ 

	becomes

		+-------------+
		|  /\ An image|
		| /  \----+   |
		|/    \   |   |
		+-------------+ 

