Adds list's index evaluation when interpolating variables

Allows variable interpolation in lists defined as integers in keys:

```
Variable:
{"my_value": ["one", "two"]}

Access with key:
'my_value.0' = 'one'
```

```
Variable:
{
  "my_value": [
    {"key": "My internal value!"}
  ]
}

Access with key:
'my_value.0.key' = 'My internal value!'
```