Support choice of debuggers with new GAFFER_DEBUGGER environment variable.
Support choice of debuggers with new GAFFER_DEBUGGER environment variable.

andrewkaufman/gaffer,chippey/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,boberfly/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,hradec/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,hradec/gaffer,boberfly/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,hradec/gaffer,chippey/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,hradec/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,chippey/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,hradec/gaffer,appleseedhq/gaffer,chippey/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,boberfly/gaffer,boberfly/gaffer,lucienfostier/gaffer