Add UrlAutoLinkExtension #3

> GFM will autolink standard URLs, so if you want to link to a URL (instead of setting link text), you can simply enter the URL and it will be turned into a link to that URL.

### Internals

This extension turns standard URL into basic markdown style autolinks.
(ex. http://example.com will be turned into <http://example.com>).
This means rendering links will be done by Core/LinkExtension.
Skip some inline parsers if it is not necessary