Hold IJulia idle status using upcoming channel interface

[JMurph2015]

Hi! This is a little early, but I have a PR out on IJulia (JuliaLang/IJulia.jl#563) address the idle message delay which was created to help Interact.jl messages avoid getting truncated.

If the PR goes through, then the preferred way to handle this will be to use a one-length boolean channel as a semaphore, register it with IJulia, and it can be used to hold the kernel from sending 'idle' until Interact fulfills the semaphore (by putting any boolean on the channel like put!(channel, true))

This is just a heads up and we'll see if the PR goes through.