Should still transpile import statements for ES6 when `--module commonjs`

[ThatRendle]

Node.js 4.x now supports enough ES6 syntax that it is desirable to use --target ES6.

But when you specify ES6, TypeScript stops transpiling the import statements, even when the --module commonjs flag is supplied, and Node does not (and apparently will never) support the import syntax.

The combination --target ES6 --module commonjs should result in regular ES6 output but with e.g.

import * as express from "express";

transpiled to

var express = require("express");

[ThatRendle]

Appears that this is already being discussed in #4389 - closing.