Simplified check for done task.

Use simple comparison with slice of string (already used e.g. in _list()) instead of grep -c pipeline.

Closes #42

todo.sh

@@ -954,7 +954,7 @@ case $action in
         [ -z "$todo" ] && die "TODO: No task $item."
 
         # Check if this item has already been done
-        if [ $(echo $todo | grep -c "^x ") -eq 0 ] ; then
+        if [ "${todo:0:2}" != "x " ]; then
             now=$(date '+%Y-%m-%d')
             # remove priority once item is done
             sed -i.bak $item"s/^(.) //" "$TODO_FILE"