Add a difference 'd' operator to marks

[alexherbo2]

https://github.com/alexherbo2/split-object.kak/issues/2#issuecomment-482069331

[Delapouite]

As shown on this illustration we may need 2 "difference operators" to decide which parts we want to keep. (Selections from the register or the ones in the buffer)

And even a third operator to express the symmetric difference.

https://en.wikipedia.org/wiki/Symmetric_difference