Add feature to Malt to capture worker output

[maleadt]

We currently use redirect_io to capture output of the worker. This is a little fragile (has resulted in errors on Windows when redirecting to a file), and doesn't capture stack traces when the worker dies. Not capturing is not an option though, because it messes with the output. We should add a feature to Malt to make it possible to cleanly capture the output of a worker.