allow custom time format string in TextFormatter

- fixes examples in README.md that incorrectly state usage of
  RFC3339Nano format instead of RFC3339

text_formatter.go

@@ -18,8 +18,9 @@ const (
 )
 
 var (
-	baseTimestamp time.Time
-	isTerminal    bool
+	baseTimestamp          time.Time
+	isTerminal             bool
+	defaultTimestampFormat = time.RFC3339
 )
 
 func init() {
@@ -46,6 +47,9 @@ type TextFormatter struct {
 	// the time passed since beginning of execution.
 	FullTimestamp bool
 
+	// Timestamp format to use for display, if a full timestamp is printed
+	TimestampFormat string
+
 	// The fields are sorted by default for a consistent output. For applications
 	// that log extremely frequently and don't use the JSON formatter this may not
 	// be desired.
@@ -68,11 +72,14 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 
 	isColored := (f.ForceColors || isTerminal) && !f.DisableColors
 
+	if f.TimestampFormat == "" {
+		f.TimestampFormat = defaultTimestampFormat
+	}
 	if isColored {
 		f.printColored(b, entry, keys)
 	} else {
 		if !f.DisableTimestamp {
-			f.appendKeyValue(b, "time", entry.Time.Format(time.RFC3339))
+			f.appendKeyValue(b, "time", entry.Time.Format(f.TimestampFormat))
 		}
 		f.appendKeyValue(b, "level", entry.Level.String())
 		f.appendKeyValue(b, "msg", entry.Message)
@@ -103,7 +110,7 @@ func (f *TextFormatter) printColored(b *bytes.Buffer, entry *Entry, keys []strin
 	if !f.FullTimestamp {
 		fmt.Fprintf(b, "\x1b[%dm%s\x1b[0m[%04d] %-44s ", levelColor, levelText, miniTS(), entry.Message)
 	} else {
-		fmt.Fprintf(b, "\x1b[%dm%s\x1b[0m[%s] %-44s ", levelColor, levelText, entry.Time.Format(time.RFC3339), entry.Message)
+		fmt.Fprintf(b, "\x1b[%dm%s\x1b[0m[%s] %-44s ", levelColor, levelText, entry.Time.Format(f.TimestampFormat), entry.Message)
 	}
 	for _, k := range keys {
 		v := entry.Data[k]