Support for slicing images before exporting.

[TricksterGuy]

Use case: The user has an image whose dimensions won't fit a valid export option and would prefer not to split it into many bitmaps.

Possible syntax

Assuming image1.png is a 96x96 image

nin10kit --mode=sprites --bpp=X image1.png[0,0,64x64] image1.png[64,0,32x64] image1.png[0,64,64x32] image1.png[64,64,32x32]

Would chop image1.png and export the top left as a 64x64, top right as a 32x64 and bottom left as a 64x32 and bottom right as a 32x32