Complete work on Facebook Instant Article exporter

Therefore, add convenience methods to `ArticleJSON::Article` class and
update README.md and CHANGELOG.md accordingly.

CHANGELOG.md

@@ -1,7 +1,7 @@
 # Changelog
 
 ## WIP
-- Start work on Facebook Instant Article exporter
+- Add an exporter for Facebook Instant Articles
 - Add support to exporters for `caption` elements that are an empty array
 - GDoc Importer: Support `[no-caption]` text, returns empty caption for element
 - Fix AMP export of Twitter tweets

README.md

@@ -32,6 +32,12 @@ puts article.to_amp
 # get javascript libraries needed for the AMP article
 puts article.amp_exporter.amp_libraries
 
+# export article as Facebook Instant Article HTML
+puts article.to_facebook_instant_article
+
+# export article as plain text
+puts article.to_plain_text
+
 # export article as JSON
 puts article.to_json
 ```
@@ -77,9 +83,10 @@ ArticleJSON.configure do |config|
     image: ArticleJSON::Export::HTML::Elements::ScaledImage
   )
 
-  # It works the same way for custom AMP exporters:
+  # It works the same way for custom AMP, FacebookInstantArticle, or 
+  # PlainText exporters:
   config.register_element_exporters(
-    :amp,
+    :amp, # Or change this for `:facebook_instant_article` or `:plain_text`
     image: ArticleJSON::Export::AMP::Elements::ScaledImage
   ) 
 end
@@ -201,6 +208,15 @@ An example of
 the AMP HTML export for the parsed reference document can be found 
 [here](https://github.com/Devex/article_json/blob/master/spec/fixtures/reference_document_exported.amp.html).
 
+### Facebook Instant Articles
+The `FacebookInstantArticle` exporter generates a custom HTML string for a list
+of elements. An example of the Facebook Instant Article export for the parsed
+reference document can be found 
+[here](https://github.com/Devex/article_json/blob/master/spec/fixtures/reference_document_exported.html).
+
+To learn more about the Facebook Instant Article HTML format see have a look at
+the [Facebook Developer Documentation](https://developers.facebook.com/docs/instant-articles/guides/format-overview).
+
 ### Plain Text
 As the name suggests, this exporter generates a plain text version of the article.
 Rich text elements like images, embeds or even text boxes are not being rendered.

lib/article_json/article.rb

@@ -62,6 +62,18 @@ def to_amp
       amp_exporter.html
     end
 
+    # Exporter instance for FacebookInstantArticle
+    # @return [ArticleJSON::Export::FacebookInstantArticle::Exporter]
+    def facebook_instant_article_exporter
+      ArticleJSON::Export::FacebookInstantArticle::Exporter.new(elements)
+    end
+
+    # FacebookInstantArticle export of the article
+    # @return [String]
+    def to_facebook_instant_article
+      facebook_instant_article_exporter.html
+    end
+
     # Exporter instance for plain text
     # @return [ArticleJSON::Export::PlainText::Exporter]
     def plain_text_exporter

spec/article_json/article_spec.rb

@@ -53,6 +53,17 @@
     it { should eq '<p>Foo Bar</p>' }
   end
 
+  describe '#facebook_instant_article_exporter' do
+    subject { article.facebook_instant_article_exporter }
+    it { should be_a ArticleJSON::Export::FacebookInstantArticle::Exporter }
+  end
+
+  describe '#to_facebook_instant_article' do
+    subject { article.to_facebook_instant_article }
+    it { should be_a String }
+    it { should eq '<p>Foo Bar</p>' }
+  end
+
   describe '#plain_text_exporter' do
     subject { article.plain_text_exporter }
     it { should be_a ArticleJSON::Export::PlainText::Exporter }