Provide an option for configuring `xhr.timeout`

[paluh]

It seems that both xhr2 and browsers support a timeout option. Should I provide a PR which adds this option to the Request (I mean timeout :: Milliseconds) type?

[chekoopa]

Affjax previously has timeout parameter as a part of RetryPolicy, which was mistakenly removed with [d710d92] (@garyb, you weren't right, we are using them actively to control backend connections, even retries).

So, yes, that PR would be pretty neat.

[chekoopa]

Bump. 😕

We're still pinning v9.0.0 as for lack of timeouts.