UI: merge child steps console output

[flosell]

If I nest my steps, sometimes I just want to get a quick overview of how the child steps are doing without expanding everything and click through individual steps.

The aggregate output of parent steps usually isn't very helpful at the moment. Instead, the UI could just merge all the childrens outputs together, e.g.

[13:03:00][component-tests] starting test...
[13:03:00][unit-tests] starting test...
[13:03:00][unit-tests] test 1 success
[13:03:02][unit-tests] test 2 sucesss
[13:03:02][component-tests] all successful!
[13:08:16][unit-tests] test 3 timeout

Drawback: Depending on the noisiness of the build steps, this aggregate might be really helpful or totally useless.

Implementation:
This could either be implemented by the parent steps aggregating the :out result of their children (might not be easy to get human readable step names there) or in the UI, aggregating the :outputs of the parent (probably requires #123 to be able to sort outputs in a meaningful way)