Introduced "clean up hooks" 

Introduced "clean up hooks"  which are a family of functions (with prefix unrarall-clean- ) which
can perform different clean up tasks.

The basic code to parse the command line options, verify they are valid and actually execute them (untested) are in place. Sorry I didn't have any rar files to hand (I deleted them all recently with unrarall :D ) to do any testing

unrarall-clean-rar and unrarall-clean-nfo have been implemented but not tested.

All other hooks (except the virtual hooks "none" and "all") have stub code only and need to be implemented and tested.

Please do some testing!