Allow aggregators to work on dimensions

[vogievetsky]

I have a numeric dimension commentLength (it is encoded as a string but all the values are numbers)

see: https://github.com/implydata/datazoo/blob/master/druid/wikipedia-index.json#L29

I want to compute the min and max of it but doing this:

    {
      "name": "MaxCommentLength",
      "type": "doubleMax",
      "fieldName": "commentLength"
    }

Gives 0.

Instead I have to do this: (I am not even mad because I ❤️ JS)

    {
      "name": "MaxCommentLength",
      "type": "javascript",
      "fieldNames": [
        "commentLength"
      ],
      "fnAggregate": "function(_c,commentLength) { return Math.max(_c,commentLength); }",
      "fnCombine": "function(a,b) { return Math.max(a,b); }",
      "fnReset": "function() { return -Infinity; }"
    }

Is it possible to have native aggregators auto parse strings as numbers if they are invoked on a dimension?

PS. the motivation for getting the min and max are contentious dimension controls like this:

[vogievetsky]

This is now possible