Add a default queryset macro

[ellmetha]

Description

Right now the only way to customize the default queryset of a specific model is to override the ::default_queryset class method as follows:

class MyModel < Marten::Model
  def self.default_queryset
    super.filter(foo: "bar")
  end
end

It would be more convenient for simple cases (like in the above example) to rely on a ::default_queryset macro allowing to define the default queryset in a very simple way:

class MyModel < Marten::Model
  default_queryset { filter(foo: "bar") }
end