Sometimes it can be very strange or long have to define default values
for config accessors as blocks, specially when config is a simple value
like 1, true, :symbol. This commit adds the ability to specify those
values by just passing a ` default` option when defining the accessor.

It also makes config accessor's interface similar to other Rails
methods like `class_attribute`, which also has the instance_reader,
instance_writer and instance_accessor options.