Add dynamic Blacklight configuration to support curator-defined fields

- add Spotlight::Catalog mixin for applying dynamic configuration to the
  upstream CatalogController

- add Spotlight::BlacklightConfiguration model to store configuration
  options and merge those options with the upstream configuration

- add exhibit#edit_metadata_fields (fixes #7) for selecting fields to
  display on the show and index pages
i18n all the things
Merge pull request #70 from sul-dlss/blacklight-dynamic-configuration

Add dynamic Blacklight configuration to support curator-defined fields