Display meta data in admin table

[stevegrunwell]

Version 0.1.0 collects meta data, but doesn't do anything with it. Future versions should display this in the WP_List_Table display.

To do this effectively, a "Screen Options" tab should be added (using standard WordPress APIs) to show/hide columns. A new filter will also need to be introduced in order to control what meta data keys are eligible to appear in the table.