Generate Pipeline models from official v1.0 Pipeline schema

[shiroyasha]

In this PR:

• I'm taking the v1.0 YAML schema, and converting it to pkg/pipelines/models.go.
• The generated models are by default added to the package main, I'm transforming this with sed to package pipelines.
• Introducing a pipelines.LoadFromYAML(<path>) method.

To test out the loading mechanism, you can run:

spc evaluate change-in --input test/fixtures/hello.yml

If everything runs fine, the output will print the loaded pipeline structure.