Mediator Pattern

https://github.com/khaled-hamam/ts-design-patterns/blob/3880c68d6bbca0a74459b97601887d5d8328db23/ROADMAP.md#L12-L17

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###### This issue was generated by [todo](https://todo.jasonet.co) based on a `TODO` comment in 3880c68d6bbca0a74459b97601887d5d8328db23 when #42 was merged. cc @khaled-hamam.