Conform to API spec for Sub Reads and Updates

[jptreen]

The API Docs specify either an ID or an email are allowed for fetching and updating a subscriber. This client doesn't allow a subscriber_id in either case. I've fixed that.

I attempted to update the test suite to test out the regex and the new control flow, but I discovered that about 40% of the tests are currently failing, so I guess this is a WIP? I've left it alone (I did, however, update the test requirements file as it was missing a dependency).

Fetch: https://developers.mailerlite.com/docs/subscribers.html#fetch-a-subscriber

GET https://connect.mailerlite.com/api/subscribers/(:id or :email)

https://developers.mailerlite.com/docs/subscribers.html#update-a-subscriber

PUT https://connect.mailerlite.com/api/subscribers/(:id)

• I've updated the regex to allow an int, and pulled the regexes out of the methods and popped them at the top of the class.
• The 'update' method of subscribers was using the POST, which is also used by the 'create' method, which I've updated to better reflect the API spec.

[jptreen]

Added in a refactor for parsing query params to improve code reuse, and fixed segments.py so you can now view subscriber listing for segments.