Add ability to wrap new subloggers

SubloggerHook is a function that when defined, is passed any new
sublogger that is created. This allows hclog users to wrap any sublogger
in similar functionality as the original, as well as adjust any
sublogger parameters dynamically.

intlogger.go

@@ -85,6 +85,8 @@ type intLogger struct {
 
 	// create subloggers with their own level setting
 	independentLevels bool
+
+	subloggerHook func(sub Logger) Logger
 }
 
 // New returns a configured logger.
@@ -151,6 +153,7 @@ func newLogger(opts *LoggerOptions) *intLogger {
 		independentLevels: opts.IndependentLevels,
 		headerColor:       headerColor,
 		fieldColor:        fieldColor,
+		subloggerHook:     opts.SubloggerHook,
 	}
 	if opts.IncludeLocation {
 		l.callerOffset = offsetIntLogger + opts.AdditionalLocationOffset
@@ -166,13 +169,21 @@ func newLogger(opts *LoggerOptions) *intLogger {
 		l.timeFormat = opts.TimeFormat
 	}
 
+	if l.subloggerHook == nil {
+		l.subloggerHook = identityHook
+	}
+
 	l.setColorization(opts)
 
 	atomic.StoreInt32(l.level, int32(level))
 
 	return l
 }
 
+func identityHook(logger Logger) Logger {
+	return logger
+}
+
 // offsetIntLogger is the stack frame offset in the call stack for the caller to
 // one of the Warn, Info, Log, etc methods.
 const offsetIntLogger = 3
@@ -774,7 +785,7 @@ func (l *intLogger) With(args ...interface{}) Logger {
 		sl.implied = append(sl.implied, MissingKey, extra)
 	}
 
-	return sl
+	return l.subloggerHook(sl)
 }
 
 // Create a new sub-Logger that a name decending from the current name.
@@ -788,7 +799,7 @@ func (l *intLogger) Named(name string) Logger {
 		sl.name = name
 	}
 
-	return sl
+	return l.subloggerHook(sl)
 }
 
 // Create a new sub-Logger with an explicit name. This ignores the current
@@ -799,7 +810,7 @@ func (l *intLogger) ResetNamed(name string) Logger {
 
 	sl.name = name
 
-	return sl
+	return l.subloggerHook(sl)
 }
 
 func (l *intLogger) ResetOutput(opts *LoggerOptions) error {

logger.go

@@ -299,6 +299,13 @@ type LoggerOptions struct {
 	// logger will not affect any subloggers, and SetLevel on any subloggers
 	// will not affect the parent or sibling loggers.
 	IndependentLevels bool
+
+	// SubloggerHook registers a function that is called when a sublogger via
+	// Named, With, or ResetNamed is created. If defined, the function is passed
+	// the newly created Logger and the returned Logger is returned from the
+	// original function. This option allows customization via interception and
+	/// wrapping of Logger instances.
+	SubloggerHook func(sub Logger) Logger
 }
 
 // InterceptLogger describes the interface for using a logger