Bitlyshortener

* Improved the readme.

* [breaking] Removed `vanitize` parameter. It is now effectively a default with a value of True. It had to be removed because `vanitize=False` seems to no longer be supported by Bitly. All users must immediately update to this version, failing which some invalid short URLs will be generated.

* Switched to using the `GITHUB_REF` environment variable for parsing the current release version number in `setup.py`. This allows not having to define the version number redundantly in the file.

* Fixed a bug affecting Citi URLs and possibly some others.

* Improve readme.

* Allow the `vanitize=True` option for `Shortener` class. This allows returning custom vanity domains whenever available. Examples of custom vanity domains are `amzn.to` and `cnn.it`. Refer to the updated examples in the readme. The default is `vanitize=False`, implying that the `j.mp` domain is always used as before.
* Use a smaller default max cache size of 256. The previous default was 2048.