Add subtitle field to Storefront

functions.php

<?php

// Display subtitle after the header for posts and pages
add_action( 'storefront_single_post', 'storefront_add_subtitle_after_header', 15 );
add_action( 'storefront_page', 'storefront_add_subtitle_after_header', 15 );
function storefront_add_subtitle_after_header() {
	the_subtitle( "<h2 class='subtitle'>", "</h2>" );
}

// Display subtitle in the header for posts
function storefront_post_header() { ?>
	<header class="entry-header">
	<?php
	if ( is_single() ) {
		storefront_posted_on();
		the_title( '<h1 class="entry-title" itemprop="name headline">', '</h1>' );
		the_subtitle( "<h2 class='subtitle'>", "</h2>" );
	} else {
		if ( 'post' == get_post_type() ) {
			storefront_posted_on();
		}
		the_title( sprintf( '<h1 class="entry-title" itemprop="name headline"><a href="%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h1>' );
		the_subtitle( "<h2 class='subtitle'>", "</h2>" );
	}
	?>
	</header><!-- .entry-header -->
	<?php
}

// Display subtitle in the header for pages
function storefront_page_header() { ?>
	<header class="entry-header">
		<?php
		storefront_post_thumbnail( 'full' );
		the_title( '<h1 class="entry-title" itemprop="name">', '</h1>' );
		the_subtitle( "<h2 class='subtitle'>", "</h2>" );
		?>
	</header><!-- .entry-header -->
	<?php
}