Add unique ID for each heading

[RadhiFadlillah]

Fix for #18. Now heading with same text :

# General

## General

### General

will rendered with unique ID for each heading :

<h1>
	<a id="general" class="anchor" href="#general" aria-hidden="true" rel="nofollow">
		<span class="octicon octicon-link"></span>
	</a>General
</h1>
<h2>
	<a id="general-1" class="anchor" href="#general-1" aria-hidden="true" rel="nofollow">
		<span class="octicon octicon-link"></span>
	</a>General
</h2>
<h3>
	<a id="general-2" class="anchor" href="#general-1-1" aria-hidden="true" rel="nofollow">
		<span class="octicon octicon-link"></span>
	</a>General
</h3>

[dmitshur]

Thanks for the PR. I'll try to find time this weekend to review.