Add config option for using fixed view on preview endpoint.

The view can be specified by setting the preview.view config option. An
example is shown below:

    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.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Change configuration option for better namespacing

Session-based data are namespaced under `session.NS.data` where `NS` is either
`defaults` or a named session. Named session are not yet supported, but this
ensure forward compatibility of the options. Each key in `data` corresponds to
a resource available for that session, such as `preview`. The value corresponds
to the custom request options or data used when requesting the resource.

Signed-off-by: Byron Ruth <b@devel.io>