Provide Blacklight configuration support for linking field values to facets

Fixes #575
    config.add_index_field 'format', :label => 'Format:', :link_to_search =>  true
    config.add_index_field 'author_display', :label => 'Author:', :link_to_search =>  :author_facet

I think the logic to render the link e.g.

```
link_to render_field_value(v), search_action_url(add_facet_params(field, value, {}))
```

probably needs to be pulled into a helper that can be overridden. But maybe that's a phase 2 concern?