Using shell from config if specified

Don't execute extra command if it's not needed

main.go

@@ -31,13 +31,13 @@ func main() {
 		logger.Fatalf("Failed to allocate pty: %s", err)
 	}
 
-	shellStr, err := loginshell.Shell()
+	shellStr := conf.Shell
+	if shellStr == "" {
+		loginShell, err := loginshell.Shell()
 		if err != nil {
 			logger.Fatalf("Failed to ascertain your shell: %s", err)
 		}
-
-	if conf.Shell != "" {
-		shellStr = conf.Shell
+		shellStr = loginShell
 	}
 
 	os.Setenv("TERM", "xterm-256color") // controversial! easier than installing terminfo everywhere, but obviously going to be slightly different to xterm functionality, so we'll see...