Added description output for heading type.

array(
	'name' => __( 'Heading with description', 'text-domain' ),
	'type' => 'heading',
	'id'   => 'heading_desc',
	'desc' => __('This is an optional description directly under this heading', 'text-domain')
),
Updated heading type with desc option
Merge pull request #514 from Neejoh/master

Added description for heading type