ADDED: command to pipe output of Prolog goals to Emacs buffers

ADDED: command to pipe output of Prolog goals to Emacs buffers

* sweep.c (sweep_open_channel(), sweep_fd_open()): new helper
functions for obtaining Prolog streams from Emacs pipe buffers.
* sweep.pl (sweep_async_goal/2, sweep_interrupt_async_goal/2): new
predicates.
* sweeprolog.el (sweeprolog-async-goal): new command, executes a goal
in a separate thread and redirects its output to a buffer with mode..
(sweeprolog-async-goal-output-mode): new major mode, derived from
Compilation mode.
(sweeprolog-mode-map, sweeprolog-prefix-map, sweeprolog-menu): bind
sweeprolog-async-goal.
* README.org (Executing Prolog Asynchronously): new manual section.