Add ability to add a baseUrl per environment to aurelia.json

**I'm submitting a feature request**

* **Operating System:**
Windows 10

* **Node Version:**
6.10.0

* **NPM Version:**
3.10.10

* **Aurelia CLI Version**
CLI 0.27.0

* **Language:**
TypeScript 2.3.2

**Current behavior:**
Currently, there is no way to change the baseUrl per deployment environment.

Our current work around is to bruteforce the line using powershell to modify, during the release process to each server.

**Expected/desired behavior:**
Ability to provide baseUrl per environment in aurelia.json file, or in environment.js file. Failing that, then ability to pass in a value via CLI parameter, so that the value can more easily be substituted on deployment

* **What is the motivation / use case for changing the behavior?**
Make it easier and more versatile to be able to deploy the same build definition to different environments such as in a CI/CD pipeline.

**Example:**
See below for the workaround we are currently having to do in our store multiple value in aurelia.json and then use Powershell to change the value.