Improve search results

Summary of changes:
- Apply the same pipeline operations to the query tokens. that is applied to
the index tokens. Before we were not removing stop words from query
tokens.
- Replace uniq with native js Set. uniq was changing position of
  elements.
- When building the index, include both stemmed and non-stemmed versions
  of tokens.
- When querying, assuming the last token in incomplete and do a prefix
  search. Add the prefix search results to the search results (if not
  already present).

Author: knubie