Export the Hook type

In order for users to implement their own hooks, they need access to the
Hook type.

hook.go

@@ -35,20 +35,22 @@ var (
 	_callerSkip = 3
 )
 
-// A hook is executed each time the logger writes an Entry. It can modify the
+// A Hook is executed each time the logger writes an Entry. It can modify the
 // entry, but must not retain references to the entry or any of its
 // contents. Returned errors are written to the logger's error output.
-type hook func(*Entry) error
+//
+// Hooks implement the Option interface.
+type Hook func(*Entry) error
 
 // apply implements the Option interface.
-func (h hook) apply(m *Meta) {
+func (h Hook) apply(m *Meta) {
 	m.Hooks = append(m.Hooks, h)
 }
 
 // AddCaller configures the Logger to annotate each message with the filename
 // and line number of zap's caller.
 func AddCaller() Option {
-	return hook(func(e *Entry) error {
+	return Hook(func(e *Entry) error {
 		if e == nil {
 			return errHookNilEntry
 		}
@@ -78,7 +80,7 @@ func AddCaller() Option {
 // or above a given level. Keep in mind that this is (relatively speaking) quite
 // expensive.
 func AddStacks(lvl Level) Option {
-	return hook(func(e *Entry) error {
+	return Hook(func(e *Entry) error {
 		if e == nil {
 			return errHookNilEntry
 		}

hook_test.go

@@ -72,10 +72,10 @@ func TestHookAddStacks(t *testing.T) {
 func TestHooksNilEntry(t *testing.T) {
 	tests := []struct {
 		name string
-		hook hook
+		hook Hook
 	}{
-		{"AddStacks", AddStacks(InfoLevel).(hook)},
-		{"AddCaller", AddCaller().(hook)},
+		{"AddStacks", AddStacks(InfoLevel).(Hook)},
+		{"AddCaller", AddCaller().(Hook)},
 	}
 	for _, tt := range tests {
 		assert.NotPanics(t, func() {

meta.go

@@ -34,7 +34,7 @@ import (
 type Meta struct {
 	Development bool
 	Encoder     Encoder
-	Hooks       []hook
+	Hooks       []Hook
 	Output      WriteSyncer
 	ErrorOutput WriteSyncer