Refactor control add_item, facilitate adding to existing sections, fix default control values

By refactoring the logic to construct the args passed into `$wp_customize->add_setting` and `$wp_customize->add_control`, less duplicated logic is in the controls. 

Also, the prefix for the generated section ID has been removed, facilitating the use of previously-added section IDs for customizer groups, to append the controls to those sections.