Added Prefix Function

A frequent use of Tries - and one of the primary benefits of the Trie structure - is the ability to find the longest prefix match.  Here I add a "prefix" function to only return the longest match in a more efficient manner than storing the entire list of prefixes, then selecting the longest match (which, now that I've seen the implementation of the prefixes function, would always be the last item in the list).  

I'm working towards using this package for bulk find replace.  Would there be any interest in adding such a thing to this package?  Let me know.