Allow getting Sub from a prefix of pflags key(s)

* It seems common enough that flags may be defined from PFlags be
  retrieved as a sub Viper similar to getting it from Yaml or other
  configuration structures. This allows flags to be bound one-to-one
  with how a config file may be structured.
* This is partially based on #331 for the find boolean flag
* fixes #368

flags_test.go

@@ -45,7 +45,7 @@ func TestBindFlagValueSet(t *testing.T) {
 
 func TestBindFlagValue(t *testing.T) {
 	var testString = "testing"
-	var testValue = newStringValue(testString, &testString)
+	var testValue = newStringValue(testString)
 
 	flag := &pflag.Flag{
 		Name:    "testflag",

viper.go

@@ -612,7 +612,7 @@ func GetViper() *Viper {
 func Get(key string) interface{} { return v.Get(key) }
 func (v *Viper) Get(key string) interface{} {
 	lcaseKey := strings.ToLower(key)
-	val := v.find(lcaseKey)
+	val := v.find(lcaseKey, true)
 	if val == nil {
 		return nil
 	}
@@ -885,9 +885,12 @@ func (v *Viper) BindEnv(input ...string) error {
 // Given a key, find the value.
 // Viper will check in the following order:
 // flag, env, config file, key/value store, default.
-// Viper will check to see if an alias exists first.
-// Note: this assumes a lower-cased key given.
-func (v *Viper) find(lcaseKey string) interface{} {
+// Viper will then check in the following order:
+// flag, env, config file, key/value store.
+// Lastly, if no value was found and flagDefault is true, and if the key
+// corresponds to a flag, the flag's default value is returned.
+//
+func (v *Viper) find(lcaseKey string, flagDefault bool) interface{} {
 
 	var (
 		val    interface{}
@@ -1003,6 +1006,18 @@ func (v *Viper) find(lcaseKey string) interface{} {
 	}
 	// last item, no need to check shadowing
 
+	// it could also be a key prefix, search for that prefix to get the values from
+	// pflags that match it
+	sub := make(map[string]interface{})
+	for key, val := range v.pflags {
+		if flagDefault && strings.HasPrefix(key, lcaseKey) {
+			sub[strings.TrimPrefix(key, lcaseKey+".")] = val.ValueString()
+		}
+	}
+	if len(sub) != 0 {
+		return sub
+	}
+
 	return nil
 }
 
@@ -1020,7 +1035,7 @@ func readAsCSV(val string) ([]string, error) {
 func IsSet(key string) bool { return v.IsSet(key) }
 func (v *Viper) IsSet(key string) bool {
 	lcaseKey := strings.ToLower(key)
-	val := v.find(lcaseKey)
+	val := v.find(lcaseKey, false)
 	return val != nil
 }
 

viper_test.go

@@ -224,9 +224,8 @@ func initDirs(t *testing.T) (string, string, func()) {
 //stubs for PFlag Values
 type stringValue string
 
-func newStringValue(val string, p *string) *stringValue {
-	*p = val
-	return (*stringValue)(p)
+func newStringValue(val string) *stringValue {
+	return (*stringValue)(&val)
 }
 
 func (s *stringValue) Set(val string) error {
@@ -587,7 +586,7 @@ func TestBindPFlagsNil(t *testing.T) {
 
 func TestBindPFlag(t *testing.T) {
 	var testString = "testing"
-	var testValue = newStringValue(testString, &testString)
+	var testValue = newStringValue(testString)
 
 	flag := &pflag.Flag{
 		Name:    "testflag",
@@ -623,7 +622,7 @@ func TestBoundCaseSensitivity(t *testing.T) {
 	assert.Equal(t, "blue", Get("eyes"))
 
 	var testString = "green"
-	var testValue = newStringValue(testString, &testString)
+	var testValue = newStringValue(testString)
 
 	flag := &pflag.Flag{
 		Name:    "eyeballs",
@@ -864,6 +863,31 @@ func TestSub(t *testing.T) {
 	assert.Equal(t, (*Viper)(nil), subv)
 }
 
+func TestSubPflags(t *testing.T) {
+	v := New()
+
+	// same as yamlExample, without hobbies
+	v.BindPFlag("name", &pflag.Flag{Value: newStringValue("steve"), Changed: true})
+	v.BindPFlag("clothing.jacket", &pflag.Flag{Value: newStringValue("leather"), Changed: true})
+	v.BindPFlag("clothing.trousers", &pflag.Flag{Value: newStringValue("denim"), Changed: true})
+	v.BindPFlag("clothing.pants.size", &pflag.Flag{Value: newStringValue("large"), Changed: true})
+	v.BindPFlag("age", &pflag.Flag{Value: newStringValue("35"), Changed: true})
+	v.BindPFlag("eyes", &pflag.Flag{Value: newStringValue("brown"), Changed: true})
+	v.BindPFlag("beard", &pflag.Flag{Value: newStringValue("yes"), Changed: true})
+
+	subv := v.Sub("clothing")
+	assert.Equal(t, v.Get("clothing.pants.size"), subv.Get("pants.size"))
+
+	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, (*Viper)(nil), subv)
+
+	subv = v.Sub("missing.key")
+	assert.Equal(t, (*Viper)(nil), subv)
+}
+
 var hclWriteExpected = []byte(`"foos" = {
   "foo" = {
     "key" = 1