small change to make file extension optional in addto

Hey Gina - 

I recently started using multiple lists to keep track of things and I found it frustrating that I always had to include the file extension when adding a task, like so... 

    todo.sh addto bucket.txt hike Appalachian trail

...but the extension was not required when listing items : 

    todo.sh lf bucket

So I added a tiny patch that allows the extension to be optional when using addto. 

Cheers,
Corinne.