Each cache miss will add a new word to either the format cache or the
substring cache. I can look at the size of the caches to determine the
number of misses rather than keep track of this with a new variable.
I don't care about the order of words for indexing, and I don't need
duplicates either. Formatting to a unique set of words should make
things a little faster.
This keeps the index up-to-date with the library. Tracks are indexed as
they are scanned, then the current filter string is run through the
index again to update it.
