add side-effects such as input/output

[Kacarott]

For reference

JohanWiltink: " I think threading stdin and stdout through source is the correct way to do I/O anyway - can use a Monad for that"

[JohanWiltink]

magic functions is another alternative, but they're considered magic for a reason.

threading stdin / stdout / stderr might allow for redirection at JS / OS level.