Implement += and -= for goods

[DavoudTaghawiNejad]

using self.destroy and self-create is unintuitive users prefer += and -=

self['cookies'] += 5

[rht]

Disagreed. self.destroy and self.create are self-descriptive, and additionally are cross-language. Additionally, they should be used carefully (a destroy should be accompanied by a create and vice versa) to make sure the system is closed.