bundle off targets, not environments

[chmontgomery]

So that bundle results are more flexible, we should bundle based off targets, e.g. "debug", "release", etc instead of environments. This way, an app can include the correct set of files based off a runtime configuration. For example, if the app is running in production but you specify a debug flag, we want to return the "debug" src files. This will require a new environment variable to key off of instead of process.env.NODE_ENV