Extract tile and subtitle from initial paragraphs

Medium guides each post to have a Title and Subtitle. They are rendered
as the first two paragraphs: H3 and H4 respectively. If they exist, a
new PageConverter class extracts them and sets them on the page.

However, they aren't required. If the first two paragraphs aren't H3
and H4, the PageConverter falls back to using the first paragraph as
the title, and setting the subtitle to blank.

The remaining paragraphs are passed into the ParagraphConverter as
normal.

spec/classes/page_converter_spec.cr

@@ -0,0 +1,97 @@
+require "../spec_helper"
+
+include Nodes
+
+describe PageConverter do
+  it "sets the title and subtitle if present" do
+    paragraphs = Array(PostResponse::Paragraph).from_json <<-JSON
+      [
+        {
+          "text": "Title",
+          "type": "H3",
+          "markups": [],
+          "href": null,
+          "iframe": null,
+          "layout": null,
+          "metadata": null
+        },
+        {
+          "text": "Subtitle",
+          "type": "H4",
+          "markups": [],
+          "href": null,
+          "iframe": null,
+          "layout": null,
+          "metadata": null
+        }
+      ]
+    JSON
+
+    page = PageConverter.new.convert(paragraphs)
+
+    page.title.should eq "Title"
+    page.subtitle.should eq "Subtitle"
+  end
+
+  it "sets the title to the first paragraph if no title" do
+    paragraphs = Array(PostResponse::Paragraph).from_json <<-JSON
+      [
+        {
+          "text": "Not a title",
+          "type": "P",
+          "markups": [],
+          "href": null,
+          "iframe": null,
+          "layout": null,
+          "metadata": null
+        }
+      ]
+    JSON
+    page = PageConverter.new.convert(paragraphs)
+
+    page.title.should eq "Not a title"
+    page.subtitle.should eq nil
+  end
+
+  it "calls ParagraphConverter to convert the remaining paragraph content" do
+    paragraphs = Array(PostResponse::Paragraph).from_json <<-JSON
+      [
+        {
+          "text": "Title",
+          "type": "H3",
+          "markups": [],
+          "href": null,
+          "iframe": null,
+          "layout": null,
+          "metadata": null
+        },
+        {
+          "text": "Subtitle",
+          "type": "H4",
+          "markups": [],
+          "href": null,
+          "iframe": null,
+          "layout": null,
+          "metadata": null
+        },
+        {
+          "text": "Content",
+          "type": "P",
+          "markups": [],
+          "href": null,
+          "iframe": null,
+          "layout": null,
+          "metadata": null
+        }
+      ]
+    JSON
+
+    page = PageConverter.new.convert(paragraphs)
+
+    page.nodes.should eq [
+      Paragraph.new([
+        Text.new("Content"),
+      ] of Child),
+    ]
+  end
+end