Enable MCE Format Tool in WP

functions.php

// Add format dropdown for MCE.
add_filter( 'mce_buttons_2', 'my_mce_editor_buttons' );
function my_mce_editor_buttons( $buttons ) {
  array_unshift( $buttons, 'styleselect' );
  return $buttons;
}
// Add SMALL and LEAD (from Bootstrap) text options.
add_filter( 'tiny_mce_before_init', 'my_mce_before_init' );
function my_mce_before_init( $settings ) {
  $style_formats = array(
  	array(
        'title' => 'Small Text',
        'inline' => 'small'
    	),
  	array(
        'title' => 'Lead Block',
        'block' => 'div',
        'classes' => 'lead'
    	)
  );
  $settings['style_formats'] = json_encode( $style_formats );
  return $settings;
}