Support before/after in DSL

[niknetniko]

The before attribute is actually used by people, e.g.

"before":  {
    "python": {
        "data": "import random; random.seed(7247)"
    }
},

This is currently not supported in the DSL, but it would be fairly easy to to do. Once we do, we can point more people towards the DSL (which has better error messages in general).