prolog-shell: Simple command line Prolog interpreter

Since Prolog isn't the most popular language, define a small
interactive interpreter that users or site administartors can play
around with by downloading the Gerrit WAR file and executing:

  java -jar gerrit.war prolog-shell

Change-Id: I0474b59e6ace24e6e6bfda58f574add3b2bf79e0

Documentation/pgm-prolog-shell.txt

@@ -0,0 +1,57 @@
+prolog-shell
+============
+
+NAME
+----
+prolog-shell - Simple interactive Prolog interpreter
+
+SYNOPSIS
+--------
+[verse]
+'java' -jar gerrit.war 'prolog-shell' [-s FILE.pl ...]
+
+DESCRIPTION
+-----------
+Provides a simple interactive Prolog interpreter for development
+and testing.
+
+OPTIONS
+-------
+-s::
+	Dynamically load the Prolog source code at startup,
+	as though the user had entered `['FILE.pl'].` into
+	the interepter once it was running. This option may
+	be supplied more than once to load multiple files.
+
+EXAMPLES
+--------
+Define a simple predicate and test it:
+
+====
+	$ cat >simple.pl
+	food(apple).
+	food(orange).
+	^D
+
+	$ java -jar gerrit.war prolog-shell -s simple.pl
+	Gerrit Code Review 2.2.1-84-ge9c3992 - Interactive Prolog Shell
+	based on Prolog Cafe 1.2.5 (mantis)
+	         Copyright(C) 1997-2009 M.Banbara and N.Tamura
+	(type Ctrl-D or "halt." to exit, "['path/to/file.pl']." to load a file)
+
+	{consulting /usr/local/google/users/sop/gerrit2/gerrit/simple.pl ...}
+	{/usr/local/google/users/sop/gerrit2/gerrit/simple.pl consulted 99 msec}
+
+	| ?- food(Type).
+
+	Type = apple ? ;
+
+	Type = orange ? ;
+
+	no
+	| ?-
+====
+
+GERRIT
+------
+Part of link:index.html[Gerrit Code Review]