Add WithCaller(), a generalized AddCaller() (uber-go#806)

Fixes uber-go#702.

logger_test.go

@@ -339,7 +339,12 @@ func TestLoggerAddCaller(t *testing.T) {
 		options []Option
 		pat     string
 	}{
+		{opts(), `^undefined$`},
+		{opts(WithCaller(false)), `^undefined$`},
 		{opts(AddCaller()), `.+/logger_test.go:[\d]+$`},
+		{opts(AddCaller(), WithCaller(false)), `^undefined$`},
+		{opts(WithCaller(true)), `.+/logger_test.go:[\d]+$`},
+		{opts(WithCaller(true), WithCaller(false)), `^undefined$`},
 		{opts(AddCaller(), AddCallerSkip(1), AddCallerSkip(-1)), `.+/zap/logger_test.go:[\d]+$`},
 		{opts(AddCaller(), AddCallerSkip(1)), `.+/zap/common_test.go:[\d]+$`},
 		{opts(AddCaller(), AddCallerSkip(1), AddCallerSkip(3)), `.+/src/runtime/.*:[\d]+$`},

options.go

@@ -87,10 +87,17 @@ func Development() Option {
 }
 
 // AddCaller configures the Logger to annotate each message with the filename
-// and line number of zap's caller.
+// and line number of zap's caller.  See also WithCaller.
 func AddCaller() Option {
+	return WithCaller(true)
+}
+
+// WithCaller configures the Logger to annotate each message with the filename
+// and line number of zap's caller, or not, depending on the value of enabled.
+// This is a generalized form of AddCaller.
+func WithCaller(enabled bool) Option {
 	return optionFunc(func(log *Logger) {
-		log.addCaller = true
+		log.addCaller = enabled
 	})
 }