Add Article::Label resource (#511)

Per the [API docs](https://developer.zendesk.com/api-reference/help_center/help-center-api/article_labels/), we are able to list / add / remove labels from articles. This adds support for those endpoints to this client.

Author: codealchemy

lib/zendesk_api/resources.rb

@@ -228,6 +228,12 @@ class Vote < Resource; end
     has_many Vote
     class Translation < Resource; end
     has_many Translation
+    class Label < DataResource
+      include Read
+      include Create
+      include Destroy
+    end
+    has_many Label
   end
 
   class TopicSubscription < Resource

spec/live/article_spec.rb

@@ -11,6 +11,14 @@
     expect(article.translations.map(&:locale)).to include("fr")
   end
 
+  it "can have labels", :vcr do
+    label = article.labels.create!(name: "ruby-client-test-label")
+
+    expect(article.labels.map(&:name)).to include(label.name)
+  ensure # Cleanup
+    label&.destroy
+  end
+
   describe "creating articles within a section" do
     def valid_attributes
       { :name => "My Article", user_segment_id: nil, permission_group_id: 2801272, title: "My super article" }