Support disposal for router

[gaearon]

I propose router has a dispose() method that stop sending events to run and cancels any pending transactions for good. Once a router is disposed, you can't use it anymore.

One use-case for this is non-global routers (although personally I don't need this).

My use case is first-class support for hot reloading for routes. This is already possible with react-hot-loader and a bit of trickery in index.js:

// some irrelevant code removed, see #606 instead

It is however slightly inconvenient to maintain isDisposed and effectively old routers keep hanging in memory potentially even reacting to location changes. Adding a dispose method to router would solve this.

[gaearon]

On a second thought, all I need is to be able to change routes mid flight.

[gaearon]

@mjackson @rpflorence

How do you feel about me proceeding with #606? I can add tests and docs if you don't mind having this. This enables a better hot loading experience and can also be useful for things like #547 where people want to tear down the router.

[ryanflorence]

I just merged 6417285 which is kinda related

[gaearon]

Yup, thanks! I'll rebase.

[mjackson]

@gaearon Yes, please! Let's follow up in #606