Add Encoder opt to emit arrays on multiple lines (#203)

A new Encoder option emits arrays with more than one line on multiple lines.
This is off by default and toggled with `ArraysWithOneElementPerLine`.

For example:

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

Becomes:

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

Fixes #200

parser_test.go

@@ -652,7 +652,7 @@ func TestTomlValueStringRepresentation(t *testing.T) {
 			"[\"gamma\",\"delta\"]"},
 		{nil, ""},
 	} {
-		result, err := tomlValueStringRepresentation(item.Value)
+		result, err := tomlValueStringRepresentation(item.Value, "", false)
 		if err != nil {
 			t.Errorf("Test %d - unexpected error: %s", idx, err)
 		}