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

[DanielePalombo]

=== 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

[johnnyshields]

:unique is a misleading name for this option. I would think that if unique: true we should generate a new (unique) short URL for the input.

I would either do unique: false to use the existing, or call it something else like use_existing: true, reuse: true, etc.