refactor: combined task output to single stream to retain order

[shreddedbacon]

closes #220 (need testing)

[tobybellwood]

This doesn't appear to work as expected:

Given the post-rollout task

    - 
      run:
        name: errors and output
        command: echo "test1" && echo "test2" 1>&2 && echo "test3" && echo "test4" 1>&2 && exit 1
        service: cli

Prior behaviour:

Failed to execute task `errors and output` due to reason `Error returned: command terminated with exit code 1`
*** Task STDOUT ***
 test1
test3
 
 *** STDOUT Ends ***
*** Task STDERR ***
 test2
test4
 
 *** STDERR Ends ***

PR behavoiur:

Failed to execute task `errors and output` due to reason `Error returned: command terminated with exit code 1`
*** Task output ***
 test1
test3
 
 *** output ends ***

[shreddedbacon]

I did some runs with this and got differing results between build runs where sometimes stderr is before stdout, and others where stdout is before stderr.

[tobybellwood]

retested working - echo only tasks run too quickly to discern order