Add formatter class and associated unit tests

tests/_data/formatter/markdown2html.target1.html

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+<p>This is a Markdown document.
+These lines belong in the same paragraph.
+Markdown usually ignores single line breaks.</p>
+
+<pre><code>This is an indented code segment.
+All Markdown variants will recognize it.
+</code></pre>
+
+<p><code>This is a fenced code segment.
+Only Markdown Extra will recognize it.</code></p>
+
+<p>This is a <a href="foobar.html">link</a>.
+This is an <img src="foobar.jpg" alt="image" />.</p>
+
+<ul><li>This is a list.</li>
+<li>It has two items.</li>
+</ul>