New puzzle: `Light Up', by James H.

Also in this checkin (committed by mistake - I meant to do it
separately), a behind-the-scenes change to Slant to colour the two
non-touching classes of diagonals in different colours. Both colours
are set to black by default, but configuration by way of
SLANT_COLOUR_* can distinguish them if you want.

[originally from svn r6164]

list.c

@@ -23,6 +23,7 @@ extern const game dominosa;
 extern const game fifteen;
 extern const game flip;
 extern const game guess;
+extern const game lightup;
 extern const game mines;
 extern const game net;
 extern const game netslide;
@@ -43,6 +44,7 @@ const game *gamelist[] = {
     &fifteen,
     &flip,
     &guess,
+    &lightup,
     &mines,
     &net,
     &netslide,