Add support for +sed format (sed -f)

bin/mkx

     #
     # Print list of valid templates
     #
+    echo sed
     echo pya
     echo shellfu
     echo Bash
             echo ''
             echo ''
             ;;
+        sed)
+            echo "#!/bin/sed -f"
+            echo ''
+            echo ''
+            ;;
         lua|Lua)
             echo "#!$(which lua)"
             echo ''