Allow customize_loaded_components filter to remove posts component

[westonruter]

Currently there is one customize_loaded_components filter which handles the injection of the WP_Customize_Posts instance into WP_Customize_Manager:

add_filter( 'customize_loaded_components', array( $this, 'filter_customize_loaded_components' ), 100, 2 );

There should be another customize_loaded_components filter that runs earlier at priority 0 which amends posts to the list of components. Then the later filter can check to see of posts is in the array and then only instantiate WP_Customize_Posts if it is present.

This would allow plugins to use a more standard way to selectively disable the posts component.

[kienstra]

Pull Request

@westonruter,
Could you please review this pull request when you have a chance? It sets a new filter add_posts_to_customize_loaded_components. This adds 'posts' to the array of components in the Customizer. And WP_Customize_Posts is only instantiated if 'posts' is still present. So a later filter may remove 'posts', to disable it.