Respect the type specific show field config

[jcoyne]

If you add configuration like:

    config.for_display_type 'Person' do
      config.add_show_field 'name_tsim', label: 'Name'
    end

Then, that config should be used to draw the pages that have documents
with that type.

[coveralls]

Coverage increased (+0.02%) to 94.684% when pulling f14f1f0 on show-field-config into a43a0b1 on master.