Add authentication_handlers

Hey @claudiofullscreen, would love your thoughts.

This PR allows for the following:

```ruby
Yt.configure do |config|
  config.client_id = Rails.application.secrets.yt_client_id
  config.client_secret = Rails.application.secrets.yt_client_secret
  config.api_key = Rails.application.secrets.yt_api_key
  config.authentication_handlers << Proc.new {|account| User.where(youtube_id: account.id).update_all(access_token: account.access_token)}
end
```