Bug: vuex-undo-redo subscribes once per component

[caugner]

I'm trying to trace down performance issues with regard to the undo functionality provided by this plugin.

When adding a console.log statement to created() here:

https://github.com/anthonygore/vuex-undo-redo/blob/6e34a39718320fbf956fb505ed3b13857c9678ce/src/plugin.js#L17-L28

I noticed that this callback is actually called several times, basically once for each created component, which leads to Vuex accumulating hundreds of subscribers.

I'm not sure how to fix this though.

[dpeckham]

Yes, I'm curious on this one.