Use square brackets instead of angle brackets in GFM

Escape angle brackets in GFM is not easy, so use square brackets.

Before:
![screen shot 2013-12-16 at 22 25 19](https://f.cloud.github.com/assets/75448/1754719/26f17e08-6657-11e3-8c5f-fb2398cac1d1.png)

After:
![screen shot 2013-12-16 at 22 27 00](https://f.cloud.github.com/assets/75448/1754720/30466856-6657-11e3-90e6-068ed92a4763.png)


Github Flavored Markdown:
```markdown
1. Fork it ( http://github.com/<my-github-username>/example/fork )
```

Expect:
```html
<li>Fork it (
<a href="http://github.com/&lt;my-github-username&gt;/example/fork">
http://github.com/&lt;my-github-username&gt;/example/fork
</a> )</li>
```

Actual:
```html
<li>Fork it ( <a href="http://github.com/">
http://github.com/</a><my-github-username>/example/fork
)</my-github-username></li>
```

Use square bracket:
```html
<li>Fork it (
<a href="http://github.com/[my-github-username]/example/fork">
http://github.com/[my-github-username]/example/fork
</a> )</li>
```