rename f.appendKeyValue to printKeyValue

printKeyValue is working similar like printColored, not using
any fields of TextFormatter, should be a util func instead of
a method of TextFormatter.

Signed-off-by: Derek Che <drc@yahoo-inc.com>

text_formatter.go

diff --git a/text_formatter.go b/text_formatter.go
index 78e7889..bbf6110 100644
--- a/text_formatter.go
+++ b/text_formatter.go
@@ -59,12 +59,12 @@
 		printColored(b, entry, keys)
 	} else {
 		if !f.DisableTimestamp {
-			f.appendKeyValue(b, "time", entry.Time.Format(time.RFC3339))
+			printKeyValue(b, "time", entry.Time.Format(time.RFC3339))
 		}
-		f.appendKeyValue(b, "level", entry.Level.String())
-		f.appendKeyValue(b, "msg", entry.Message)
+		printKeyValue(b, "level", entry.Level.String())
+		printKeyValue(b, "msg", entry.Message)
 		for _, key := range keys {
-			f.appendKeyValue(b, key, entry.Data[key])
+			printKeyValue(b, key, entry.Data[key])
 		}
 	}
 
@@ -104,7 +104,7 @@
 	return true
 }
 
-func (f *TextFormatter) appendKeyValue(b *bytes.Buffer, key, value interface{}) {
+func printKeyValue(b *bytes.Buffer, key, value interface{}) {
 	switch value.(type) {
 	case string:
 		if needsQuoting(value.(string)) {