Issue title: Return the completed item_user matrix
 Issue body: Dear Ben. I am using `implicit` for my own project. Thank you for making such a good package! My problem is how can I get a completed item_user matrix or user_item matrix after done `model.fit()`? Thank you!
 Comments: 
 Comment 0: The fitted model object will have user_factors and item_factors arrays which can be matrix multiplied together:

import numpy as np
user_item_matrix = np.dot(model.user_factors,model.item_factors.T)
 Comment 1: Thank you very much for the solution! @rwinklerwilkes 
{'source': 'GithubIssues', 'repo_name_with_owner': 'benfred/implicit', 'issue_id': 407939541, 'repo_stars': 3003.0}