Allow disabling timestamp in non-tty output.

This is useful if the output is already being piped to a logger
daemon that will prefix the timestamp by itself (eg: on heroku).
diff --git a/text_formatter.go b/text_formatter.go
index fc0a408..6df95ea 100644
--- a/text_formatter.go
+++ b/text_formatter.go
@@ -32,8 +32,9 @@
 
 type TextFormatter struct {
 	// Set to true to bypass checking for a TTY before outputting colors.
-	ForceColors   bool
-	DisableColors bool
+	ForceColors      bool
+	DisableColors    bool
+	DisableTimestamp bool
 }
 
 func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
@@ -53,7 +54,9 @@
 	if isColored {
 		printColored(b, entry, keys)
 	} else {
-		f.appendKeyValue(b, "time", entry.Time.Format(time.RFC3339))
+		if !f.DisableTimestamp {
+			f.appendKeyValue(b, "time", entry.Time.Format(time.RFC3339))
+		}
 		f.appendKeyValue(b, "level", entry.Level.String())
 		f.appendKeyValue(b, "msg", entry.Message)
 		for _, key := range keys {