Add an option to whether to include previously liked items or not

[ita9naiwa]

Issue #131 says recommending already seen items will be good in some situations.

[srcolinas]

I think it would be possible to improve this commit by making the user_items argument optional and assuming that if it is None it means we do not want to ignore previously liked items.

[ita9naiwa]

@srcolinas As far as I know, user-item matrix is required to build recommemdation to user.

[srcolinas]

@ita9naiwa wouldn't it be possible to build the recommendations based on item_factors and user_factors?

[ita9naiwa]

sorry, its the case of nearest neighbour methods

Factor based models can generate recommenendations without user-item matrix

[benfred]

@ita9naiwa thanks for the PR!