Implement `parallel' (just alias of run_after)

[mizzy]

I've added the `parallel' config keyword to execute commands parallel. This is the implementation of #107 .

In this implementation, parallel is just an alias of run_after . But I think parallel is more understandable.

I've tested this code with this pipeline.yml.

pipeline:
  - name: command 1
    command: sleep 3
  - name: parallel commands
    parallel:
      - name: parallel command 1
        command: sleep 5
      - name: parallel command 2
        command: sleep 5
  - name: command 2
    command: sleep 3

Please check it.

[takahi-i]

LGTM