Use instance method to configure setting

This is useful to change ES server or index by request context like following example.

```ruby
module Kibana::Sinatra
  class Web
    def elasticsearch_url
      case request.path
      when /foo/
        "http://foo.example.com:9200"
      when /bar/
        "http://bar.example.com:9200"
      else
        "http://localhost:9200"
      end
    end
  end
end
```