Support ES6-style compact object literals for multiselect

Say I have this JSON object and want to pare it down to just the ids and first names:

```json
{
"values": [
{"id": 1, "first": "Bill", "last": "Gates"},
{"id": 2, "first": "Larry", "last": "Page"}
]
}
```

I can do that with this expression: `values[*].{id:id, first:first}`.

Since `{x:x, y:y}` is such a common pattern, ES6 introduced [compact object literal notation][1] to simplify this. In ES6, `{x,y}` is equivalent to `{x:x, y:y}`.

I don't believe `{x,y}` is valid in JMESPath, so it would be a boon (and intuitive to JS users) to add this to the spec.

[1]: http://www.benmvp.com/learning-es6-enhanced-object-literals/