Allow a function to run on the roottask and launch the kernel via @async

[mkitti]

@roflmaostc requested a mechanism to use IJulia with JavaCall.jl:
JuliaInterop/JavaCall.jl#145

1. For JavaCall to work without setting the environmental variable JULIA_COPY_STACKS=1, we need it to run on the initial Julia task ( Base.roottask).
2. Could we setup a mechanism the IJulia kernel is run via @async on a new Task and an arbitrary function is run on the root Task? This would be similar to the current configuration of the Julia REPL where the backend is running on the root Task and the front end is running on another asynchronous Task?