Add support for synonyms

[rmind]

• We want to add an API to support user-defined synonyms associated with the index.
• Initially support query-time synonyms, e.g. search for "ewe" would result in search for "ewe" or "sheep".
• Consider index-time synonyms where tokens are converted to some chosen generic form.
• Consider using and integrating with a synonyms database for English. We want to handle, at least, cases like "calibre" vs "caliber", "defense" vs "defence", "tech" vs "technology", etc.