This supports using `--json` to get configuration information in a
structured way.
The objects we return have the following schema:
```
{
name: string;
value: string?;
secret: bool;
}
```
In the case of `pulumi config` when --show-secrets is not passed, and
there are secret values, the `value` property of the object for that
configuration value will not be set. This differs from the normal
rendering where we show `[secret]`.
Contributes To #496
