Twenty Nineteen: Consider adding a filter for the featured image color filter functionality.

[kjellr]

Originally raised by @hvianna and @grapplerulrich in this GitHub thread:

​https://github.com/WordPress/twentynineteen/issues/722

To make it easier for child themes to disable the featured image filter built into Twenty Nineteen, it might be helpful to add a filter for that functionality. Something along the lines of:

function twentynineteen_image_filters_enabled() {
	return apply_filters( 'twentynineteen_image_filters_enabled', 0 !== get_theme_mod( 'image_filter', 1 ) );
}