ADDED: command for sending a goal to top-level from any buffer

ADDED: command for sending a goal to top-level from any buffer

* sweeprolog.el (sweeprolog-top-level): extract buffer setup logic to...
(sweeprolog-top-level-buffer): new function.
(sweeprolog-top-level-send-string): new function, used by...
(sweeprolog-top-level-send-goal): new command, reads a goal from the
minibuffer and sends it to a Prolog top-level buffer.
(sweeprolog-mode-map): bind it to C-c C-q.
(sweeprolog-menu): add "Send Goal to Top-level" entry.
* README.org ("Sending Goals to the Top-level"): new manual section.