String Interpolation for advanced configuration

I'm not sure if this is still within the spirit of node-convict but I needed a simple interpolation for a project. Here's what it does (I can provide/write tests here but it wasn't entirely clear to me where a feature test should go):

If your json file is as follows with the corresponding schema definition -- I'm assuming both properties are "*" types here.
```
{
"property1": "${property2[0]}"
"property2": [ "test" ]
}
```
This will allow you to interpolate the value in property1 key (assuming the load and validation steps have already been performed) so you get:

```
{
 "property1" : "test"
}
```