Add support for custom checks in DSL

[niknetniko]

This adds support for custom checks in the DSL:

- tab: 'Test'
  contexts:
    - testcases:
        - statement: 'test()'
          return_raw:
            value: "'hallo'"
            evaluator: "custom"
            language: "python"
            file: "test.py"
            name: "evaluate_test"
            arguments: ["'yes'", "5", "set([5, 5])"]

As a reminder, custom checks are available for textual output (e.g. stdout, stderr) and return values. For return values, only return_raw is supported (not return, as we cannot know if an object is a value or a custom check).

Fixes #391.