added sum function to shellscript version

shellmath.sh

@@ -8,3 +8,9 @@ function mul { echo $(($1*$2)); }
 function div { echo $(($1/$2)); }
 function mdl { echo $(($1%$2)); }
 function randr { echo $(($1+$RANDOM%$2)); }
+function sum {
+  for i in $@; do
+	acc=$((acc+i))
+  done
+  echo $acc
+}