Initial state

Hey @cmather 

This is a simple change that just flags our first state with `{initial:true}`. This means the user can check if `history.state.initial` at any time to know if the back button will break out of their site.

Realistically, we should probably be doing more with state (e.g. letting users set it in route filter and storing it, allowing them to access it via `Location.state` and `Router.current().state` etc), but this solves my problem in the meantime.

Let me know what you think.