I would like JavaScript filters to be able to work on multiple columns

I would like to do something like this:

```
{
  "type" : "javascript",
  "dimensions" : ["dim1", "dim2"]
  "function" : "function(d1, d3) { return d1 === d2 }"
}
```

Here is a thread about how useful this would be: https://groups.google.com/forum/#!topic/druid-user/-MEsJOUX_5I
