allow custom time format string in TextFormatter

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

text_formatter_test.go

@@ -3,8 +3,8 @@ package logrus
 import (
 	"bytes"
 	"errors"
-
 	"testing"
+	"time"
 )
 
 func TestQuoting(t *testing.T) {
@@ -33,5 +33,29 @@ func TestQuoting(t *testing.T) {
 	checkQuoting(true, errors.New("invalid argument"))
 }
 
+func TestTimestampFormat(t *testing.T) {
+	checkTimeStr := func(format string) {
+		customFormatter := &TextFormatter{DisableColors: true, TimestampFormat: format}
+		customStr, _ := customFormatter.Format(WithField("test", "test"))
+		timeStart := bytes.Index(customStr, ([]byte)("time="))
+		timeEnd := bytes.Index(customStr, ([]byte)("level="))
+		timeStr := customStr[timeStart+5 : timeEnd-1]
+		if timeStr[0] == '"' && timeStr[len(timeStr)-1] == '"' {
+			timeStr = timeStr[1 : len(timeStr)-1]
+		}
+		if format == "" {
+			format = time.RFC3339
+		}
+		_, e := time.Parse(format, (string)(timeStr))
+		if e != nil {
+			t.Errorf("time string \"%s\" did not match provided time format \"%s\": %s", timeStr, format, e)
+		}
+	}
+
+	checkTimeStr("2006-01-02T15:04:05.000000000Z07:00")
+	checkTimeStr("Mon Jan _2 15:04:05 2006")
+	checkTimeStr("")
+}
+
 // TODO add tests for sorting etc., this requires a parser for the text
 // formatter output.