GFM header IDs fail if header contains span markup #391
Description
On GitHub proper (and kramdown prior to 1.13.0), the following MD:
### `code-span`
...results in the following HTML:
<h3 id="code-span"><code>code-span</code></h3>
Now, though, it results in something like:
<h3 id="-18"><code>code-span</code></h3>
(...assuming this wasn't the first code-span header in the page.)
This is bad, and also does not match what GitHub does.