Add support for getting array index in GetValue

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, "dat", "2")
```

The initial data object still needs to be a `map[string]interface{}`
type.

pkg/data/values.go

@@ -1,5 +1,9 @@
 package data
 
+import (
+	"strconv"
+)
+
 func RemoveValue(data map[string]interface{}, keys ...string) (interface{}, bool) {
 	for i, key := range keys {
 		if i == len(keys)-1 {
@@ -18,18 +22,38 @@ func GetValueN(data map[string]interface{}, keys ...string) interface{} {
 	return val
 }
 
-func GetValue(data map[string]interface{}, keys ...string) (interface{}, bool) {
+func GetValue(data interface{}, keys ...string) (interface{}, bool) {
 	for i, key := range keys {
 		if i == len(keys)-1 {
-			val, ok := data[key]
-			return val, ok
+			if dataMap, ok := data.(map[string]interface{}); ok {
+				val, ok := dataMap[key]
+				return val, ok
+			}
+			if dataSlice, ok := data.([]interface{}); ok {
+				return itemByIndex(dataSlice, key)
+			}
+		}
+		if dataMap, ok := data.(map[string]interface{}); ok {
+			data, _ = dataMap[key]
+		} else if dataSlice, ok := data.([]interface{}); ok {
+			data, _ = itemByIndex(dataSlice, key)
 		}
-		data, _ = data[key].(map[string]interface{})
 	}
 
 	return nil, false
 }
 
+func itemByIndex(dataSlice []interface{}, key string) (interface{}, bool) {
+	keyInt, err := strconv.Atoi(key)
+	if err != nil {
+		return nil, false
+	}
+	if keyInt >= len(dataSlice) || keyInt < 0 {
+		return nil, false
+	}
+	return dataSlice[keyInt], true
+}
+
 func PutValue(data map[string]interface{}, val interface{}, keys ...string) {
 	if data == nil {
 		return

pkg/data/values_test.go

@@ -56,14 +56,44 @@ func TestGetValue(t *testing.T) {
 			name: "get index of slice",
 			data: map[string]interface{}{
 				"parent": map[string]interface{}{
-					"children": []string{
+					"children": []interface{}{
 						"alice",
 						"bob",
 						"eve",
 					},
 				},
 			},
 			keys:        []string{"parent", "children", "2"},
+			wantValue:   "eve",
+			wantSuccess: true,
+		},
+		{
+			name: "index is too big",
+			data: map[string]interface{}{
+				"parent": map[string]interface{}{
+					"children": []interface{}{
+						"alice",
+						"bob",
+						"eve",
+					},
+				},
+			},
+			keys:        []string{"parent", "children", "3"},
+			wantValue:   nil,
+			wantSuccess: false,
+		},
+		{
+			name: "index is negative",
+			data: map[string]interface{}{
+				"parent": map[string]interface{}{
+					"children": []interface{}{
+						"alice",
+						"bob",
+						"eve",
+					},
+				},
+			},
+			keys:        []string{"parent", "children", "-3"},
 			wantValue:   nil,
 			wantSuccess: false,
 		},