[AC-318] Support AdHoc vs. Production keys in config.json file

Thanks to ALOY-680, you can differentiate between AdHoc and Appstore/Play Store builds in code (such as in alloy.js or your controllers). However, you cannot differentiate between such builds in the config.json. We use different endpoints for AdHoc builds (Installr/TestFlight beta builds) vs our production releases. Currently, we have to manually copy & paste the contents of the config.json file to support such builds. This has resulted in mistakes. While we can use external tooling, like a grunt script, to manipulate the files, it would be best if Alloy itself supported something like:

{
    "env:development": {"url":"https://dev.example.com"},
    "env:test":{"url":"https://test.example.com"},
    "env:production":{"url":"https://production.example.com"},
    "dist:adhoc":{"url":"https://adhoc.example.com"},
    "dist:store":{"url":"https://production.example.com"},
}

Given that we need this support for both iOS and Android, the key can't be env:adhoc. In the above proposal, dist:adhoc and dist:store would need to override env:production Ultimately, it would be great to use AdHoc/Store variables in TSS and XML files as well.