markdown library adds em HTML tags into urls

Boo.

My fork of markdown-js will put em tags into the middle of urls.

```
<a href="http://news.indiewebcamp.com/post/innercircle.aok.io/open_garden_any_good" class="u-syndication" rel="syndication">
```

becomes

```
<a href="http://news.indiewebcamp.com/post/innercircle.aok.io/open<em>garden</em>any_good" class="u-syndication" rel="syndication">
```
