Add alt code for 2004/arachnid for h/j/k/l keys

Now vi users can more easily enjoy this entry without getting
frustrated! Obviously that includes myself. Use j/k/l/h keys instead.
There's some circular fun in the README.md about this based on what the
judges' originally wrote (that made me laugh out loud because it's
true - I did get lost quite easily though I did solve it ...
eventually).

2004/arachnid/Makefile

@@ -114,8 +114,8 @@ OBJ= ${PROG}.o
 DATA= arachnid.txt
 TARGET= ${PROG}
 #
-ALT_OBJ=
-ALT_TARGET=
+ALT_OBJ= ${PROG}.alt.o
+ALT_TARGET= ${PROG}.alt
 
 
 #################
@@ -137,6 +137,8 @@ ${PROG}: ${PROG}.c
 alt: data ${ALT_TARGET}
 	@${TRUE}
 
+${PROG}.alt: ${PROG}.alt.c
+	${CC} ${CFLAGS} $< -o $@ ${LIBS}
 # data files
 #
 data: ${DATA}