Allow to different presets to share code

[pkozielecki]

As is:

For now, when we want to add preset-specific code to a stage, we need to duplicate common code for all the presets, eg.:

report:
    default:
        - sh:
            command: "bundle exec danger"
        - copy_artifacts:
            path: $(ENV:BITRISE_DEPLOY_DIR)
    staging:
        - sh:
            command: "bundle exec danger"
        - sh:
            command: "preset-specific command"
        - copy_artifacts:
            path: $(ENV:BITRISE_DEPLOY_DIR)
    clientqa:
        - sh:
            command: "bundle exec danger"
        - sh:
            command: "preset-specific command"
        - copy_artifacts:
            path: $(ENV:BITRISE_DEPLOY_DIR)_

As it should be:

There should be a way to define a common code for all the presets within a given stage:

report:
    common:
        - sh:
            command: "bundle exec danger"
        - copy_artifacts:
            path: $(ENV:BITRISE_DEPLOY_DIR)
    staging:
        - sh:
            command: "preset-specific command"
    clientqa:
        - sh:
            command: "preset-specific command"_