Add or improve include directive in action Yaml

[johhnry]

It would be nice to include pieces of yaml into another one like a mixin:

# silex_coins.yml

silex_coins:
  index: 500
    commands:
      add_silex_coins:
        path: "silex_client.commands.user.silex_coins.AddSilexCoinsCommand"
          parameters:
            amount:
              value: 5

and inserting the step in the yaml:

# publish.yml

publish:
  # ...
  steps:
    silex_coins: !include "silex_coins.yml"