Add the ability to use an index number to get an item out of a slice.

The number will still be a string representation of an integer. For
example, to get the last element from a nested slice like:

```
data := map[string]interface{}{
    "data": []interface{}{
        "first",
        "second",
        "third",
    },
}
```

Use keys like

```
val := GetValues(data, "data", "2")
```

Adds package comments and extra test cases

Restores GetValue to it's original interface to avoid breaking changes.