Add field 'column' parameter, and the concept of field value display.

You can now set admin post-listing columns with an extra field
parameter, `'column' => true,`. If you want to dictate what position
the column is, use `'column' => array( 'position' => 2 ),`. If you
want to dictate the column title (instead of using the field `'name'`
value), use `'column' => array( 'name' => 'My Column' ),`. If you need
to specify the column display callback, set the `'display_cb'` parameter
to [a callback function](https://github.com/WebDevStudios/CMB2/wiki/Field-Parameters#render_row_cb).

includes/CMB2_hookup.php

@@ -41,6 +41,13 @@ class CMB2_hookup extends CMB2_Hookup_Base {
 	 */
 	protected $columns = array();
 
+	/**
+	 * The object type we are performing the hookup for
+	 * @var   string
+	 * @since 2.0.9
+	 */
+	protected $object_type = 'post';
+
 	/**
 	 * Constructor
 	 * @since 2.0.0