Abort: destroy streams when killing processes

I noticed that sometimes builds would continue after being aborted.
The launched process would be dead but its children would live on and
continue to write to the stream.
Merge abort--kill-streams (#64) into master

Abort: destroy streams when killing processes

I noticed that sometimes builds would continue after being aborted.
The launched process would be dead but its children would live on and
continue to write to the stream.