Add jsdom instance reference to global

[Li0liQ]

Having a reference to jsdom will allow to access its API (see #26).

[ValentinH]

Being able to access the jsdom instance is really nice. @rstacruz any idea when this can be merged?

[modosc]

i setup a fork here:

https://github.com/modosc/global-jsdom

with it you can access the instance via global.$jsdom

[odinho]

So this project seems dead @rstacruz? This functionality would be good, so we don't have to duplicate all the code just to get access to reconfigure.