[TextFormatter] Speed up. Fprintf is changed to buffer.Write*

As Fprintf is slower than buffer.WriteString, it's replaced
to faster call.

Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>

formatter_bench_test.go

@@ -1,6 +1,7 @@
 package logrus
 
 import (
+	"fmt"
 	"testing"
 	"time"
 )
@@ -45,6 +46,15 @@ var largeFields = Fields{
 	"entries":   "yeah",
 }
 
+var errorFields = Fields{
+	"foo": fmt.Errorf("bar"),
+	"baz": fmt.Errorf("qux"),
+}
+
+func BenchmarkErrorTextFormatter(b *testing.B) {
+	doBenchmark(b, &TextFormatter{DisableColors: true}, errorFields)
+}
+
 func BenchmarkSmallTextFormatter(b *testing.B) {
 	doBenchmark(b, &TextFormatter{DisableColors: true}, smallFields)
 }