Allow to pass :user to Ahoy::Store through Ahoy::Tracker

[korun]

Hello @ankane, thanks for the gem.

So, we can pass :user directly to Ahoy::Tracker, but, unfortunately, it not defines one in Ahoy::Store.
This request fixes that.

This can be useful, for example, when we track some events asynchronously, without :controller, and with some additional user's parameters usage in Ahoy::Store#track_event.

[ankane]

Thanks @korun!