add {{ content_dir }} and {{ content_url }} to twig_vars

This allows you to use {{ content_dir }} or {{ content_url }} in your theme templates.

I use it for adding images (sliders, header images, logos) or other file-specific content that I still want to be accessible by the client. It is a lot easier for them if their site "lives" in the content folder.

My usage: 

```html
{% for slide in config.slides %}
  <div>
    <img src="{{ content_url }}/images/{{ slide }}">
  </div>
{% endfor %}
```

Output

```html
<div>
  <img src="http://ohdoylerules.com/images/slide-1.jpg">
</div>
<div>
  <img src="http://ohdoylerules.com/images/slide-2.jpg">
</div>
```

This makes for a universal image. Which can be used regardless of the theme I have enabled.