Expose many_cats and num_to_cat to the user

[mbcann01]

Right now, the values passed to the many_cats and num_to_cat parameters of the cb_add_summary_stats() function can only be changed by interacting directly with the cb_add_summary_stats() function, which isn't exported, and therefore, not exposed to users. we need to be able to pass these parameters down from the codebook() function.

After you make this change, describe it in README. Possibly in the "Add column attributes to the data frame you want to document with a codebook" section of Example 2.