Add configurable session attributes

[MauriceBrg]

Add user_info_to_session_attr_mapping as a new optional parameter. It can be used to add user information to the current session after authentication. It's also possible to rename the Cognito attributes here, so they appear under a different name in the session, e.g.:

{"sub": "user_id", "email": "email", "username": "username"}

Here, the Cognito Subscriber-ID (sub) will be added to the session as user_id. The attributes email and username are added unchanged.

The original default of only adding the email attribute is retained, but you need to explicitly add it to the mapping if you choose to use it.

Fixes #15