Add #should_render_field? support for all solr field types (index, show,
sort, search, facets).

Solr fields support :if and :unless configuration, e.g.

  config.add_index_field if: false
  config.add_index_field if: :some_helper_method
  config.add_index_field if: lambda { |solr_field, document| false }
  config.add_index_field unless: true
Merge pull request #853 from projectblacklight/if-unless

Add #should_render_field? support for all solr field types (index, show,