Ability to disable window syncing

Hi,

We're currently using a small fork of this project that disables syncing across windows. It literally just comments out one line in order to achieve this: https://github.com/funkensturm/ember-local-storage/blob/eece52d4c5ea1b528ed3e5dfc9eaa8c56be6fd3e/addon/mixins/storage.js#L49

We'd like to avoid using a fork if possible, so would this be able to be made into a global option? I'll do a PR for the change, just want to get the go-ahead from a maintainer first on the approach. I can't see any existing global options configurable via `ember-cli-build.js`, but can add that system in the PR.

Alternatively, we could use the existing `'options` hash that can be passed to `storageFor()`, but this appears to be deprecated and tagged for removal in 2.0. If we instead deprecated just the `legacyKey` property and not the whole options hash then it could be used for this config too. This feels a bit awkward though, as really these are global options, so having to specify them on every call to `storageFor()` may not be ideal.

Thoughts?

Best,
Richard