Feature: Add Support for Firestore

[ilyaigpetrov]

It's quiet easy but I don't feel my code is robust yet: https://github.com/ilyaigpetrov/telegraf-session-firebase
Either I should read more of documentation and recheck everything or someone more experienced should add this feature.

[MicxSamonte]

Bump on this @dotcypress

[KnorpelSenf]

As firestore and firebase realtime database are two distinct storage solutions, I believe it makes sense to provide individual middlewares for them. I created telegraf-session-firestore for this purpose. It is based on both this repository and @ilyaigpetrov's fork, but fixes a minor issues with the latter in case of a non-existing document (edit: the problem did not occur in real-world applications, your code was good). Also, I added TypeScript typings.
If you think these two middlewares should be unified, we can see how we can merge them. Otherwise this issue may be closed.