merge with istanbul-lib-hook

istanbul-lib-hook, and append-transform have similar functionality; I think we should figure out how to merge the two.

istanbul-lib-hook has the added advantage that it handles hooking into the `vm.createScript` method:

https://github.com/istanbuljs/istanbul-lib-hook/blob/master/lib/hook.js#L109

I'd love to get this functionality added to nyc; @jamestalmage, @gotwarlost what do you think the best option is for this? Perhaps we could keep both libraries, with append-transform as a dependency in istanbul-lib-hook?