Added completedGamesOnly property for getUserOwnedGames

[Greenylie]

Hello xDimGG, first of all nice work with this package, the code is well-written and easy to follow, as well as the docs. I had an idea for a project, which just uses completed games, so I decided to contribute to the package itself instead of filtering the results locally. This is my very first contribution, so please feel free to criticize something that doesn't feel right.

Note: I edited every modified link in the relative docs to TODO, so you can just add the new one where it's needed.