Add support of long-term tokens

[belyaev-dev]

New update added long-term access tokens for private integrations. They are the same as the access token, but has longer expire period.

Do we need to update readme? Basically we can use new token as an access_token, but specify expired_at as 5 years or less (depends on real token lifetime)

[shevernitskiy]

In a first glace, it wouldn't request any changes to codebase. You can make an a PR to clarify this moment in the readme.

[belyaev-dev]

works fine, just requires setting up expires_at manually (and filling up other fields with fake data)