JSON output support

JSON does not allow trailing commas after the last element of a list. Thus the following is legal:

``` json
{
  "a": "1", 
  "b": "2"
}
```

While this is not:

``` json
{
  "a": "1", 
  "b": "2",
}
```

Does lustache support this somehow? Would it be possible to write a function that adds a comma to it's output for all elements of a list but the last?
