<?php
function custom_tinymce_config($init)
{
	// Don't remove line breaks
	$init['remove_linebreaks'] = false; 
	// Convert newline characters to BR tags
	$init['convert_newlines_to_brs'] = true; 
	// Do not remove redundant BR tags
	$init['remove_redundant_brs'] = false;
	// Add valid tags
	$opts = '*[*]';
	$init['valid_elements'] = $opts;
	$init['extended_valid_elements'] = $opts;
	// See all configuration
	// print_r($init); exit;
	return $init;
}
add_filter('tiny_mce_before_init', 'custom_tinymce_config');