Simple interface for invaliding a list of files

Re #10, I think that would make sense. How do you feel about still adding in the leading slash and directories that are above index pages?

Also, would you want to include a helper like this? Should it be better namespaced?
I would prefer the second version since it
* doesn't change the `files` array that was passed as an argument but creates a new one and
* doesn't change the `files` array while looping over it.