Add support for BulkDelete/BulkAdd/Collection

[arvindkumarc]

Right now, in order to delete or modify or create a batch of documents to a collection, there is no simple way. All we have to do is to query, and getAllData and then perform the required operation. Eg: To delete an entire collection.

Some methods like firestore.deleteCollection(), firestore.bulkAdd(collectionName, [d1, d2]) should help to avoid multiple calls to firestore.