Expose configuration.

This came up while coordinating two different libraries that each wanted to
`RSVP.configure('onerror', someFunction)`. As the tests make clear the last
one "wins" and any prior error functions will be clobbered.

This change allows me to check for the existence of an existing
`onerror` so that my library isn't a bad citizen in the RSVP ecosystem.

Additionally, this change allows libraries to chain `onerror` calls. This
might look like:

```javascript var priorOnerror = RSVP.config.onerror;

RSVP.configure('onerror', function(error){
 // do something important here
 // then call the prior onerror handler
 priorOnerror(error);
});
```