add default parametter to HashModel.get

[b-simjoo]

Hi, I added new constant value __INCOMPLETE as default value for optional parameters and then added a new optional parameter named default to get function. When user pass value to default parameter (including None and Ellipsis) then if key not exist it will return default, otherwise it raises NotFoundError

[abrookins]

Well, this deserved an answer a long time ago! Maintainership of this repo was in flux at the time. I'm belatedly reviewing older PRs, so thank you @b-simjoo for opening the PR. After thinking about this, I'd like to keep get() 100% consistent in raising an error if an object doesn't exist with the target PK. Thanks again!