Support YAML tags

[niknetniko]

This PR adds support for YAML tags in the DSL.

• We support the standard YAML !!set (fixes #401)
• We support all TESTed types via "local data types", meaning with a single exclamation mark: !int64, !set, ... (fixes #402)

For example:

- tab: 'Test'
  contexts:
    - testcases:
        - statement: 'test()'
          return: !!set {5, 6}

Note that the return statement is not checked by the JSON Schema (as it is not capable of doing custom types), so if you put nonsense, it will crash with an exception somewhere (e.g. !int64 [5, 6] will result in a crash).