Allow specifying stdin (#360)

* Allow specifying stdin

__tests__/scripts/wait-for-input.cmd

@@ -0,0 +1,3 @@
+@echo off
+set /p var=
+echo %var%

__tests__/scripts/wait-for-input.js

@@ -0,0 +1,3 @@
+var fs = require('fs')
+var data = fs.readFileSync(0, 'utf-8')
+process.stdout.write(data)

__tests__/scripts/wait-for-input.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+
+read var
+echo $var