Set stdin in UpstreamProcess to os.Stdin (instead of /dev/null)

Fixes basecamp#16

internal/upstream_process.go

@@ -15,9 +15,11 @@ type UpstreamProcess struct {
 }
 
 func NewUpstreamProcess(name string, arg ...string) *UpstreamProcess {
+	cmd := exec.Command(name, arg...)
+	cmd.Stdin = os.Stdin
 	return &UpstreamProcess{
 		Started: make(chan struct{}, 1),
-		cmd:     exec.Command(name, arg...),
+		cmd:     cmd,
 	}
 }