ADDED: new command for exporting Prolog predicates

ADDED: new command for exporting Prolog predicates

* sweep.pl: sweep_local_predicate_export_comment/2: new predicate.
* sweeprolog.el:
- sweeprolog-read-exportable-predicate-prompt: new user option.
- sweeprolog--exportable-predicates: new buffer local variable.
- sweeprolog--colour-term-to-faces: accumulate exportable predicates.
- sweeprolog-colourise-buffer: reset sweeprolog--exportable-predicates.
- sweeprolog-export-predicate: new command.
- sweeprolog-mode-map: bind it.
- sweeprolog-menu: bind it.
- README.org: document it.