adding option "baseContext". Closes #15

This pull request intends to provide an option to set a base context using dust's **makeBase** method.

Setting the option:

```javascript
var kleiDust = require('klei-dust');

kleiDust.setOptions({
    baseContext: {
        config: {
            cdnUrl: 'https://mycdn.com'
        }
    }
});
```

Using it:

```html
<script src="{config.cdnUrl}/js/all.js"></script>
```

This way we don't need to set these config options every time, neither use `app.set` that pushes the objects to the **settings** property.