Add flexible persistable create methods with multiple new documents.

Now these methods accept inserting multiple new documents without square brackets.
  e.g.
  ```ruby
    Person.create({ title: "Mr" }, { title: "Mrs" })
    Person.create([{ title: "Mr" }, { title: "Mrs" }])

    # bang version
    Person.create!({ title: "Mr" }, { title: "Mrs" })
    Person.create!([{ title: "Mr" }, { title: "Mrs" }])
  ```