add formatting option for list of todos, update documentation

[rzach]

This should complete the changes suggested in issue #64

[rzach]

BTW the \todostyle command silently redefines styles already existing, so if you want to you can settle on a default done style (e.g., just greying it out) in the package itself which users can then redefine as I described in 1.8.3

[rzach]

@henrikmidtiby are you still considering to include this?