Added option for extract existing url from history without create new one

=== Shorten without generating new short URL (if exists long_url in history)                                                                                                                                                                                                                                                  
  client = Googl.client('user@gmail.com', 'my_valid_password')
  url = client.shorten('https://github.com/zigotto/googl')
  url.short_url
  => http://goo.gl/DWDfi                                                         
                                                                                 
  url = client.shorten('https://github.com/zigotto/googl')                       
  url.short_url                                                                  
  => http://goo.gl/Oh48S                                                         
                                                                                 
  url = client.shorten('https://github.com/zigotto/googl', unique: true)          
  url.short_url                                                                  
  => http://goo.gl/Oh48S