Fix initialization of cfgDir (#422)

It is erroring with the message `Error: configuring the main config directory: mkdir : no such file or directory` because of an inverted comparison accidentally introduced in commit f713dc6.

cmd/gmailctl/cmd/root_cmd.go

@@ -56,7 +56,7 @@ func init() {
 
 // initConfig reads in config file and ENV variables if set.
 func initConfig() {
-	if cfgDir == "" {
+	if cfgDir != "" {
 		// Use config file from the flag.
 		return
 	}