Allow getUserConfirmation callback result to cater for any type of value

[mymattcarroll]

Closes #489

We'd like to have a multi-option confirmation popup (save/discard/cancel) and need a way to pass additional details about the actions, and an action handler into getUserConfirmation.

The changes proposed will not break compatibility but will allow for any type of value returned from the callback.

[mymattcarroll]

@mjackson, any chance of getting this merged? We would really like this feature in https://github.com/FormidableLabs/redux-little-router

[mjackson]

Hey @mymattcarroll, thanks for the PR. And my apologies for the long delay following up. Can you please describe a bit more about what you're trying to do? The getUserConfirmation method is giving a lot of people trouble, so I'm currently on the look-out for a better solution.

[mymattcarroll]

@mjackson as this was quite a while ago, I don't remember the exact use case but we wanted to pass a more complex object as the result which was more than just JSON so we could not stringify it either. If you need more information, I might have to wait until the weekend to dig back into the code base we were using this lib in.

[mjackson]

No worries. I just figured I'd ask to see if you remembered off the top of your head.

We are actually planning on removing getUserConfirmation in the future because it forces you to declare a function up front when you create your history object instead of being able to define a function where you do the block. That's probably the reason you had to pass some extra information through getUserConfirmation, so you could access the context you were in when you blocked the navigation attempt. This will be a lot more straightforward in the future.

[mymattcarroll]

Yep, I think that was exactly the issue we were having. Looking forward to the update. :)