Ability to express CONCAT as an extractionFn

[vogievetsky]

This is part of Plywood/PlyQL and is currently being done using javascript

SELECT CONCAT('[', `language`, ']') FROM `wikipedia` GROUP BY 1

I would like to express this:

{
  "type": "javascript",
  "function": "function(str) { return '[' + str + ']'; }",
  "injective": true
}

as

{
  "type": "concat",
  "prefix": "[",
  "postfix": "]"
}

or

{
  "type": "sprintf",
  "expr": "[%s]"
}

or equivalent