feat: add support for environment-based build pipeline

[jeffbcross]

In #215, where AppCache/ServiceWorker support was implemented, the feature was limited because there was no way to differentiate between a build for dev and a build for prod inside lib/broccoli/angular2-app.js.

The service worker or app cache should not be installed for dev builds by default, since it would interfere with rapid change app > reload > repeat lifecycle.

I'm just opening this issue for discussion/tracking of allowing different behavior for different environments; I don't have a proposed design. Given that dev and prod builds should generally not exhibit different behavior, this design should have good constraints to discourage abuse.

[cironunes]

I'm looking into this, but didn't make much progress yet.

[hansl]

I'll close this in favor of #41. Cirocunes is already working on this.

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