include uniqueness filter in noramlizeTokens

[phanos]

typeahead.js/src/bloodhound/search_index.js

Line 136 in 662ee46

function normalizeTokens(tokens) {

I was looking at issue #183 regarding starts with logic. This seems to be a result of how the Ngram tokenizer works. I was playing around with implementing a tokenizer that would output interword tokens. It would generate a lot more tokens. I noticed normalizeTokens does not return unique list tokens. It seems like getIntersection efficiency would benefit from being fed a unique list? Would there be any downsides to this?