Update nlopes/slack vendor

vendor/github.com/nlopes/slack/logger.go

@@ -0,0 +1,53 @@
+package slack
+
+import (
+	"fmt"
+	"sync"
+)
+
+// SetLogger let's library users supply a logger, so that api debugging
+// can be logged along with the application's debugging info.
+func SetLogger(l logProvider) {
+	loggerMutex.Lock()
+	logger = ilogger{logProvider: l}
+	loggerMutex.Unlock()
+}
+
+var (
+	loggerMutex = new(sync.Mutex)
+	logger      logInternal // A logger that can be set by consumers
+)
+
+// logProvider is a logger interface compatible with both stdlib and some
+// 3rd party loggers such as logrus.
+type logProvider interface {
+	Output(int, string) error
+}
+
+// logInternal represents the internal logging api we use.
+type logInternal interface {
+	Print(...interface{})
+	Printf(string, ...interface{})
+	Println(...interface{})
+	Output(int, string) error
+}
+
+// ilogger implements the additional methods used by our internal logging.
+type ilogger struct {
+	logProvider
+}
+
+// Println replicates the behaviour of the standard logger.
+func (t ilogger) Println(v ...interface{}) {
+	t.Output(2, fmt.Sprintln(v...))
+}
+
+// Printf replicates the behaviour of the standard logger.
+func (t ilogger) Printf(format string, v ...interface{}) {
+	t.Output(2, fmt.Sprintf(format, v...))
+}
+
+// Print replicates the behaviour of the standard logger.
+func (t ilogger) Print(v ...interface{}) {
+	t.Output(2, fmt.Sprint(v...))
+}