Add config option for using fixed view on preview endpoint.

Fix #454.

The view can be specified by setting the preview.view config option. An
example is shown below:

```json
    preview:
        view:
            columns:[2]
```

or

```coffeescript
    c.config.set('preview.view', {columns: [2]})
```
In the above example, all calls to the preview endpoint will use a view
composed of only the concept with ID 2. This is an example taken from
varify to use only the sample column in calls to the preview endpoint.