Make multi-line arrays always use trailing commas (#217)

This makes ArraysWithOneElementPerLine output arrays with commas after every element.

```
A = [1,2,3]
```

Now becomes:

```
A = [
  1,
  2,
  3,
]
```

marshal.go

@@ -187,7 +187,7 @@ func (e *Encoder) QuoteMapKeys(v bool) *Encoder {
 //   A = [
 //     1,
 //     2,
-//     3
+//     3,
 //   ]
 func (e *Encoder) ArraysWithOneElementPerLine(v bool) *Encoder {
 	e.arraysOneElementPerLine = v