Added input box to filter files when list is over 5 entries long

I have updated the work done by @jswartwood in [this pull request](https://github.com/jashkenas/docco/pull/54) and implemented a file filter that will be displayed if there are more than 5 files displayed within the documentation.

I decided not to implement the scroll bar on the list as it keeps it much simpler and I think having the input filter works well enough.