Allow centering of text on paragraphs with center class

This change allows centering of text within a paragraph if the paragraph has a 'center' class, e.g.:

```html
<p class='center'>I'm centered</p>
```

or 

```html
<p class='center'><strong>I'm strong and centered</strong></p>
```

Apparently HTML5 doesn't support the \<center> tag anymore so I thought this was a better approach.