Add Sub() for Viper, which returns a branch of a Viper instance.

README.md

@@ -441,6 +441,52 @@ will be returned instead. E.g.
 GetString("datastore.metric.host") //returns "0.0.0.0"
 ```
 
+### Extract sub-tree
+
+Extract sub-tree from Viper.
+
+For example, `viper` represents:
+
+```json
+app:
+  cache1:
+    max-items: 100
+    item-size: 64
+  cache2:
+    max-items: 200
+    item-size: 80
+```
+
+After executing:
+
+```go
+subv := viper.Sub("app.cache1")
+```
+
+`subv` represents:
+
+```json
+max-items: 100
+item-size: 64
+```
+
+Suppose we have:
+
+```go
+func NewCache(cfg *Viper) *Cache {...}
+```
+
+which creates a cache based on config information formatted as `subv`.
+Now it's easy to create these 2 caches separately as:
+
+```go
+cfg1 := viper.Sub("app.cache1")
+cache1 := NewCache(cfg1)
+
+cfg2 := viper.Sub("app.cache2")
+cache2 := NewCache(cfg2)
+```
+
 ### Unmarshaling
 
 You also have the option of Unmarshaling all or a specific value to a struct, map,

viper.go

@@ -513,6 +513,18 @@ func (v *Viper) Get(key string) interface{} {
 	return val
 }
 
+// Returns new Viper instance representing a sub tree of this instance
+func Sub(key string) *Viper { return v.Sub(key) }
+func (v *Viper) Sub(key string) *Viper {
+	data, ok := v.Get(key).(map[string]interface{})
+	if !ok {
+		return nil
+	}
+	subv := New()
+	subv.config = data
+	return subv
+}
+
 // Returns the value associated with the key as a string
 func GetString(key string) string { return v.GetString(key) }
 func (v *Viper) GetString(key string) string {

viper_test.go

@@ -724,3 +724,16 @@ func TestWrongDirsSearchNotFound(t *testing.T) {
 	// been ignored by the client, the default still loads
 	assert.Equal(t, `default`, v.GetString(`key`))
 }
+
+func TestSub(t *testing.T) {
+	v := New()
+	v.SetConfigType("yaml")
+	v.ReadConfig(bytes.NewBuffer(yamlExample))
+
+	subv := v.Sub("clothing.pants")
+	assert.Equal(t, v.Get("clothing.pants.size"), subv.Get("size"))
+
+	subv = v.Sub("clothing.pants.size")
+	assert.Equal(t, subv, (*Viper)(nil))
+}
+