accomodate ember-data promises

[brenna]

Often options.content or selectedItem.content will need to be passed (instead of options or selectedItem for full functionality.

The addon internals currently don't distinguish between regular old objects and ember-data promises. It would be nice if both ordinary objects and ember-data promises could be passed in.

see https://github.com/miguelcobain/ember-cli-selectize/issues/58 for similar issue/strategies.