#18 Add modified_stamp token to storylist format

[Travis Miller]

It'd be nice if the format attribute of the storylist tag offered a token to output the modified_stamp. We have a list of recently updated stories on our home page, and we'd like to show the time each one was last updated.

The same effect can currently be achieved by using the published_stamp token (%d), but if bug 1516437 is resolved, this will break. Both tokens should be available.