Add blacklight configuration for registering additional response formats for the #index action

Options include:

- do the default
    config.index.respond_to.yaml = true

- don't render the format
    config.index.respond_to.yaml = true

- options for render
    config.index.respond_to.yaml = { layout: 'custom-layout' }

- custom proc to render
    config.index.respond_to.yaml = lambda { render text: "stuff" }

- controller method to render
    config.index.respond_to.yaml = :my_custom_yaml_serialization
Merge pull request #761 from projectblacklight/index-response-formats

Add blacklight configuration for registering additional response formats...