Refresh / flush Vuex store when Druxt Auth user logout.

[Decipher]

Is your feature request related to a problem? Please describe.
When a user logs out via the Druxt Auth integration, the Vuex stores persist.

Describe the solution you'd like
Some way to universally flush the Vuex stores that can be used by modes such as Druxt Auth.

Describe alternatives you've considered
Triggering a manual page reload after the Nuxt Auth logout call in the frontend.

Additional context
The Druxt Store already has the ability to flush resources and collections from the store: https://github.com/druxt/druxt.js/blob/develop/packages/druxt/src/stores/druxt.js#L144-L180

Other Vuex stores will also need this ability.