Master script for pipeline exec

It would be great if we could define one 'master' step inside pipeline.yml that aggregates and executes other steps. 
Maybe with a syntax like (lets say we have already defined three steps, namely 'trimming', 'mapping' and 'quantification')

``` ruby
  master_step:
    desc: Step that aggregates other sub-steps
    run:
      > trimming
      > mapping
      > quantification

```
