Add location.delta about history

[mjackson]

I think I'd prefer to have something a bit more general, like location.delta that would give us the n the user supplied to go(n). When going back, location.delta would be -1. Likewise, when going forward location.delta would be 1.

[xpepermint]

Even better :).

[ricardosoeiro]

This would be a great improvement! I'm also relying on location.action === "POP" to detect back transitions and perform a different animation, but it is not reliable because of the (less frequent) forward transitions. Is this planned?

[mjackson]

I think this would make a great addition to the library, but haven't had time to get to it. I would love it if someone else could step up and take a shot at this. It's a feature that a lot of people seem to want.

[mjackson]

This is superseded by #334. If we can figure out how to provide location.index, users can derive the delta for themselves.