Add syntax for array of hashes.

I'd like to propose this syntax for the creation of arrays of hashes. The mnemonic is an array (outside set of brackets) of hashes (inside set of brackets). Get it?

```toml
[[products]]
name = "Hammer"
sku = 738594937

[[products]]

[[products]]
name = "Nail"
sku = 284758393
color = "gray"
```

In JSON land, that would give you the following structure.

```json
{
  "products": [
    { "name": "Hammer", "sku": 738594937 },
    { },
    { "name": "Nail", "sku": 284758393, "color": "gray" }
  ]
}
```