Add formatter class and associated unit tests

tests/_data/formatter/markdown2html.source.md

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+This is a Markdown document.
+These lines belong in the same paragraph.
+Markdown usually ignores single line breaks.
+
+    This is an indented code segment.
+    All Markdown variants will recognize it.
+
+```
+This is a fenced code segment.
+Only Markdown Extra will recognize it.
+```
+
+This is a [link](foobar.html).
+This is an ![image](foobar.jpg).
+
+  - This is a list.
+  - It has two items.