Add Pocket provider

This adds support for [Pocket](https://getpocket.com).

Quite a few changes to the daemon itself were necessary as Pocket uses “a variant of OAuth 2.0”. Basically, it resembles the OAuth 1.0 protocol but uses terminology from OAuth 2.0.

To work around that, I have added a `response` object to the provider’s configuration which works similarly to the `query` object but you can use response parameters as placeholders. For example, the API returns `code` instead of `oauth_token` so the configuration file defines `"oauth_token": "{{code}}"`.

Furthermore, you can now use `oauth_token` and `oauth_token_secret` in query objects.

If you’re fine with that approach I could a few words to the documentation and adjust the tests.