add test for horizontal rule

tests/data/horizontal_rule.html

+<p>Dashes:</p>
+<hr />
+<hr />
+<hr />
+<hr />
+<pre><code>---</code></pre>
+<hr />
+<hr />
+<hr />
+<hr />
+<pre><code>- - -</code></pre>
+<p>Asterisks:</p>
+<hr />
+<p>Underscores:</p>
+<hr />
+<p>Based on <a href="http://daringfireball.net/projects/downloads/MarkdownTest_1.0.zip">the original</a> test suite.</p>
\ No newline at end of file

tests/data/horizontal_rule.md

+Dashes:
+
+---
+
+ ---
+ 
+  ---
+
+   ---
+
+	---
+
+- - -
+
+ - - -
+ 
+  - - -
+
+   - - -
+
+	- - -
+
+Asterisks:
+
+***
+
+Underscores:
+
+___
+
+Based on [the original](http://daringfireball.net/projects/downloads/MarkdownTest_1.0.zip) test suite.
\ No newline at end of file