adding Merriam-Webster dictionary & thesaurus data (#794)

Hi! I'd like to suggest Merriam-Webster's dictionary and thesaurus APIs. These are free for non-commercial use, up to 1,000 requests per day: https://dictionaryapi.com/

README.md

@@ -259,6 +259,7 @@ API | Description | Auth | HTTPS | CORS |
 ### Dictionaries
 API | Description | Auth | HTTPS | CORS |
 |---|---|---|---|---|
+| [Merriam-Webster](https://dictionaryapi.com/) | Dictionary and Thesaurus Data | `apiKey` | Yes | Unknown |
 | [Oxford](https://developer.oxforddictionaries.com/) | Dictionary Data | `apiKey` | Yes | Unknown |
 | [Wordnik](http://developer.wordnik.com) | Dictionary Data | `apiKey` | No | Unknown |
 | [Words](https://www.wordsapi.com/) | Definitions and synonyms for more than 150,000 words | `apiKey` | Yes | Unknown |