Add support for Readwise.io

[DavidMetcalfe]

Since Omnivore is dead, one of the replacement services offered up in communications from Omnivore staff was Readwise.io

I've found it to be quite a wonderful alternative, and their API seems solid, so I'd like to put forward an enhancement request to add support to reink for Readwise.

[kebot]

Yeah, also switched to Readwise Reader
but it seems Reader currently only have two APIs: save, and list at the moment: https://readwise.io/reader_api