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?
This what you had in mind? Or would you prefer something like:

```ruby
index_files = files.select { |f| File.basename(f) == 'index.html' }
files += index_files.map { |f| "#{File.dirname(f)}/" }
```

I haven't yet learned how to make these kinds of decisions.