Allow trailing commas in arrays

A configuration language should allow a trailing comma at the end of an array. The intent is unambiguous, since arrays only contain primitives.

This lets you comment elements of an array in or out:

```
plugins = [
  "hemiconducer",
  "defremulator",
  #  "carbozaxor"
]
```

One of the infuriating things about JSON as a config language is that it will explode on that array, because of the trailing comma.