Extend deps: to allow for multiple "pipelines", that run their commands in parallel

[skaldesh]

Imagine this current grml command:

...
build:
  help: build everything
  deps:
    - targetA
    - targetB.1
    - targetB.2
    - targetC
    - targetD
...

This is possible with the current stucture and would run the commands sequentially in the given order.
But it would be desirable to run commands that do not depend on each other in parallel.

Let's say, that targetA must be built before everything else, and targetB.1 must be built before targetB.2.
The following execution would be optimal:

> start targetA
< wait until finished (abort if failed)
> start targetB.1
> start targetC
> start targetD
< wait until targetB.1 finished (abort if failed)
> start targetB.2
< wait until all finished (abort if failed)