Suport for new Firestore

[ebertti]

Hi,

Firebase relaease a new service called Firestore, with a new way to store data.

https://firebase.google.com/docs/firestore/?hl=pt-br

The vuefire team has some plans to make a bind to that too?

[posva]

Duplicate of #145 😉
Next time check at least the issues first 😛