Patch: support existing woocommerce_mail_content filter

Hi,

Love your plugin, we’ve used it to debug duplicate content and layout of our woocommerce emails. However, the woocommerce_mail_content filter is not supported in the output, so if you’re doing any modifications to the emails using this official filter, they don’t show up. I don’t see a public repo, so here’s a quick patch for line 93 of functions.php:
echo $new_email->style_inline( $new_email->get_content() );
is replaced by

$contents = apply_filters( 'woocommerce_mail_content', $new_email->style_inline( $new_email->get_content() ) );
echo $contents;

Thanks!