GFM Permissive Autolinks

md4c does not support extended protocol autolinks from the Github Flavored Markdown Spec.
https://github.github.com/gfm/#extended-protocol-autolink

```
mailto:foo@bar.baz

xmpp:foo@bar.baz
```

```
<p><a href="mailto:foo@bar.baz">mailto:foo@bar.baz</a></p>
<p><a href="xmpp:foo@bar.baz">xmpp:foo@bar.baz</a></p>
```