autokey: Added 'fixed' attribute to control if the value should be changed on update

Added a new option to the `autokey` plugin to control if the key should be updated on every save.

Use cases:
- A blog with posts with fixed slugs can keep original URLs even if titles change (ie: "Javascript case study (updated)").
- A fixed slug, updated on create but editable later from the Admin UI, or regenerable if value is empty.