Add crop command about sic

[foresterre]

Implemented in #83.

[foresterre]

Thanks for noticing!

Planned for 0.9.0

Possible syntax options:

1. crop xa ya xb yb with

   • xa: top left corner x-coordinate
   • ya: top left corner y-coordinate
   • xb: bottom right corner x-coordinate
   • ya: bottom right corner y-coordinate

2. crop xa ya w h with

   • xa: bottom left corner x-coordinate
   • ya: bottom left corner y-coordinate
   • w: width of the subimage view
   • h: height of the subimage view

or variations on the options above.

Probably (1) makes the most sense as its analogical to cropping in most image editors like Paint.

Selected choice: (1).