Adding model performance in table

[DominiqueMakowski]

In traditional APA regression tables, model performance metrics (R2) are included at the bottom line (see here). We could add an extra column in to_table or to_fulltable, with an empty name, displaying the indices of fit in an extra line.

This would be quite clunky from a data analysis pipeline (if one want to extract the parameters and do something with them later on), but useful for the table to be reported "as is", which is the scope of the package.

A solution could be a parameter show_model_metrics with which one could remove this extra line and column.

[DominiqueMakowski]

Added.