Allow Yt.configuration through env variables

Instead of running an initializer like

Yt.configure do |config|
  config.scenario = :device_app
  config.client_id = '1234567890.apps.googleusercontent.com'
  config.client_secret = '1234567890'
  config.api_key = '123456789012345678901234567890'
end

you can now do the following

export YT_CLIENT_SCENARIO="device_app"
export YT_CLIENT_ID="1234567890.apps.googleusercontent.com"
export YT_CLIENT_SECRET="1234567890"
export YT_API_KEY="123456789012345678901234567890"