Add 'persistence_method' option that is called when an attribute is written to

This is useful when using an ORM like ActiveRecord where it is desirable to
set the enum value without having to additionally call `#save!()` after setting
the enum field's value via the `!` accessor writer method.

Usage:

Set the option to the desired method to be called when an attribute is written to:

`SimpleEnum.persistence = "save!"`