API: add "show_counts" arg to info
From https://github.com/pydata/pandas/issues/6939, @jorisvandenbossche made the suggestion that

> we could add a keyword argument like show_counts (or another name) to specify if you want to show the non-null counts (to be able to override the max_info_rows option for a specific info call)

scheduling for .14.1 unless someone gets to it before then.
