Support changed markers

[ddanier]

Sometimes it might be necessary to mark your object to be changed for certain things - while this information is not available through the fields itself.

Usage could be:

obj = Something()

assert not something.model_has_changed  # False

obj.model_mark_changed("some_key_you_like")

assert something.model_has_changed  # True
obj.model_changed_markers  # {"some_key_you_like"}

obj.model_unmark_changed("some_key_you_like")

assert not something.model_has_changed  # False

Example usage could be if you are using relational databases and want to mark the change of child DB structures in your parent.

[ddanier]

@simone-ebeling Could you please review the changes in #20

[ddanier]

Released with 0.6.0