Add support for onAbort argument on XHR.perform

[bpinto]

XHR.perform method creates a K.stream and emits value by binding to
some XMLHttpRequest events. That way you can be notified about this
request state changes through observable events.

As you might already guess activation and deactivation propagates
up the observables chain. For instance if one create a long chain like
Kefir.fromEvents(...).map(...).filter(...).take(...), the whole chain
will be inactive until first subscriber is added, and then it will
activate up to .fromEvents. Same for deactivation.

*Extracted from Kefir docs.

While the request is correctly aborted when the request observable is
unsubscribed, the event is not propagated to the observable chain given
they have already been unsubscribed. I'm not familiar with any solution
within Kefir realm that would allow us to be notified of such changes,
and while one could probably create an abstraction on top of
XHR.perform to run a custom code on abortion (unsubscription), it
seems weird not to provide a solution for this problem.

The proposal is to support a custom onAbort argument when calling
XHR.perform that will be executed when the request is deactivated and
subsequently aborted.