Add :merriamwebster_thesaurus support for not-found responses

[hunterstich]

When a word isn't found by MW-t, the response contains an array of similar words as show in this call: https://www.dictionaryapi.com/api/v3/references/thesaurus/json/rowel?key=<DEVKEY>

Add support to :merriamwebster_thesaurus to be able to display the list of possible word alternatives.