Add OxfordDictionary::Client#thesaurus

Oxford Dictionaries is updating their API to a new version which
includes quite a few changes:
https://developer.oxforddictionaries.com/version2

They are moving the thesaurus functionality to its own endpoint
(instead of having part of the entries endpoint).

Use this instead of `Client#entry_synonyms`, `Client#entry_antonyms`,
and `Client#entry_antonyms_synonyms`.

lib/oxford_dictionary/client.rb

@@ -7,6 +7,7 @@
 require 'oxford_dictionary/endpoints/lemmas'
 require 'oxford_dictionary/endpoints/translations'
 require 'oxford_dictionary/endpoints/sentences'
+require 'oxford_dictionary/endpoints/thesaurus'
 
 module OxfordDictionary
   # The client object to interact with
@@ -71,6 +72,14 @@ def sentence(word:, language:, params: {})
       sentence_endpoint.sentence(word: word, language: language, params: params)
     end
 
+    def thesaurus(word:, language:, params: {})
+      thesaurus_endpoint.thesaurus(
+        word: word,
+        language: language,
+        params: params
+      )
+    end
+
     private
 
     def lemma_endpoint
@@ -95,6 +104,12 @@ def sentence_endpoint
       )
     end
 
+    def thesaurus_endpoint
+      @thesaurus_endpoint ||= OxfordDictionary::Endpoints::Thesaurus.new(
+        request_client: request_client
+      )
+    end
+
     def request_client
       @request_client ||=
         OxfordDictionary::Request.new(app_id: @app_id, app_key: @app_key)

spec/client_spec.rb

@@ -94,6 +94,21 @@
     end
   end
 
+  describe '#thesaurus' do
+    subject { client.thesaurus(word: word, language: language, params: params) }
+    let(:word) { 'ace' }
+    let(:language) { 'en' }
+    let(:params) { {} }
+
+    it 'calls the Thesaurus endpoint with correct arguments' do
+      expect_any_instance_of(OxfordDictionary::Endpoints::Thesaurus).
+        to receive(:thesaurus).
+        with(word: word, language: language, params: params)
+
+      subject
+    end
+  end
+
   describe '#entry_snake_case' do
     let(:client) { described_class.new(app_id: app_id, app_key: app_key) }
     subject do