add tests for clean heading IDs

test/new/headings-id.html

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+<h3 id="heading-with-a-link">Heading with a <a href="http://github.com/">link</a></h3>
+
+<h3 id="heading-with-some-italic-text">Heading with some <em>italic text</em></h3>
+
+<h3 id="or-some-strong">Or some <strong>strong</strong></h3>
+
+<p>(which doesn&#39;t really make any difference, here)</p>
+
+<h3 id="or-even-code">Or even <code>code</code></h3>
+
+<h3 id="what-about-strikethrough">What about <del>strikethrough</del></h3>
+
+<h2 id="and-a-ref-link">And a ref <a href="/some/url" title="link to nowhere">link</a></h2>

test/new/headings-id.md

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+### Heading with a [link](http://github.com/)
+
+### Heading with some _italic text_
+
+### Or some **strong**
+(which doesn't really make any difference, here)
+
+### Or even `code`
+
+### What about ~~strikethrough~~
+
+## And a ref [link][destination]
+
+[destination]: /some/url "link to nowhere"