Request: Transfer idleTimeout to session data level

[dplynskiy]

I have requirement to have different idleTimeout/lifetime setting for different users. While I can use SetDeadline to set lifetime of session, I can't do something similar for idle timeout. I think it's good idea to transfer idleTimeout setting to session data level and make method for overwriting it for certainly session.

[alexedwards]

I think it should be possible to implement by adding a special __idleTimeout value to sessions, in a similar way to the RememberMe() method.

If anyone is interested in working on this / has time, I'm going to mark this as 'help wanted' and a PR would be appreciated.