Add option flag to support re-selecting until Return key is pressed

[m-bartlett]

Adds -C which prevents slurp from exiting immediately once the rectangle drag is released. The rectangle may be re-selected, and selection is ended by pressing the Return key. The intent of this feature is to enable users to redo a selection if the first attempt was unsatisfactory.

[heroin-moose]

Bump? This is a life safer for me.