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
```