I am completely fine with merging additional trees in my own Brocfile (i.e. not in the EmberApp helper), but perhaps we should assert with a friendly message if anything is specified as an option other than the ones we are specifically allowing?

Seems like it would be an odd source of confusion, as we essentially are saying "you can provide your own custom app/style/tests/vendor trees when calling the `EmberApp` helper by specifying them in the `tree` property in the options hash that you pass to EmberApp", but then we completely ignore them.