You would pass a block like this:

    irb(main):002:0> client = Nessus::Client.new 'http://localhost/' do |fc|
    irb(main):003:1*   fc.response :logger
    irb(main):004:1> end
    => #<Nessus::Client:0x007fabe3a07530 @connection=#<Faraday::Connection:0x007fabe3a07170 @parallel_manager=nil, @headers={"User-Agent"=>"Nessus.rb v0.1.0.beta.18"}, @params={}, @options=#<Faraday::RequestOptions (empty)>, @ssl=#<Faraday::SSLOptions verify=true>, @default_parallel_manager=nil, @builder=#<Faraday::RackBuilder:0x007fabe3a06a40 @handlers=[Faraday::Response::Logger]>, @url_prefix=#<URI::HTTP:0x007fabe28d52d0 URL:http://localhost/>, @proxy=nil>>

For more info see https://github.com/lostisland/faraday.