Different aurelia.json options dependent on environment

**I'm submitting a feature request**

* **Library Version:**
0.29.0

**Please tell us about your environment:**
* **Operating System:**
10

* **Node Version:**
6.10.0

* **NPM Version:**
3.10.10

* **Browser:**
all

* **Language:**
all

**Current behavior:**
Related to issue #403

We can now specify, with the useAbsolutePath flag, a baseUrl, which has allowed us to call the vendor-bundle file from another domain, and ensures the vendor bundle file calls the correct app-bundle, and doesnt look at the root of the domain, which has been great!

**Expected/desired behavior:**

* **What is the expected behavior?**
We would like to be able to change this base url based upon the environment that we have built.

For example:

```
"build": {
"targets": [
{
"id": "web",
"displayName": "Web",
"output": "scripts",
"useAbsolutePath": true,
"baseUrl": {
"dev": "http://localhost:9000/scripts",
"stage": "http://appsdev.nottscc.gov.uk/ContactACouncillorAurelia/scripts",
"prod": "http://ws84-0029.nottscc.gov.uk/ContactACouncillor/scripts",
"default": "http://ws84-0029.nottscc.gov.uk/ContactACouncillor/scripts"
}
}
]
```
My guess is that dev, stage and prod would relate to the environments that you can build in, and default speaks for itself.

* **What is the motivation / use case for changing the behavior?**
This would eliminate our need to manually change the json file for each. We have managed to do a powershell script to find and replace the line itself, we're investigating doing it with a gulp task, but this change would really help, and make automation seemless.