Adding class for centering in markdown

With this you can easily center text by use of markdown's *IALs* (Block Inline Attribute Lists).


**Example:**
{:.center}
Your text

**Will result in:**
<p class="center">Your text</p>

So CSS will "text-align:center".
Merge pull request #92 from ACCakut/patch-1

Adding class for centering in markdown