Use square brackets instead of angle brackets in GFM

Escape angle brackets in GFM is not easy, so use square brackets.

Github Flavored Markdown:
1. Fork it ( http://github.com/<my-github-username>/example/fork )

Expect:
<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:
<li>Fork it ( <a href="http://github.com/">
http://github.com/</a><my-github-username>/example/fork
)</my-github-username></li>

Use square bracket:
<li>Fork it (
<a href="http://github.com/[my-github-username]/example/fork">
http://github.com/[my-github-username]/example/fork
</a> )</li>
Merge pull request #2760 from sanemat/fix/fork-url

Use square brackets instead of angle brackets in GFM