Plugin Customizer, full source at https://github.com/soderlind/plugin-customizer

customize_register-priority.php

<?php
$customize_register_priority = 9;
add_action( 'wp_loaded', function() {
	add_action( 'customize_register', array( $this, 'customizer_plugin_settings' ) );
}, $customize_register_priority );

init-previewer-script.php

<?php
/**
 * Parameters sent to the previewer script is:
 *  url		The custom permalink to the default template. The last part, newsletter in the exmple below
 *  		will translate to newsletter.php in the templates folder.
 */
$default_url = array(
	'url' => home_url( 'plugin-customizer/templates/newsletter/?' . PLUGIN_CUSTOMIZER_PARAM_NAME . '=' . PLUGIN_CUSTOMIZER_PARAM_VALUE ),
);
/**
 * Add a template to a section. key = section name, value = url to template.
 *
 * The last part, title and content in the exmple below, will translate to title.php
 * and content.php in the templates folder.
 */
$section_urls = array(
	// 'newsletter_title_section'   => home_url( 'plugin-customizer/templates/title/' ),
	// 'newsletter_content_section' => home_url( 'plugin-customizer/templates/content/' ),
);
wp_add_inline_script(
	$handle,
	sprintf( 'PluginCustomizer.init( %s, %s );',
		wp_json_encode( $default_url ),
		wp_json_encode( $section_urls )
	),
	'after'
);

rewrite-rule.php

<?php
function rewrite_rule( $wp_rewrite ) {
	 $new_rules = array(
		  'plugin-customizer/templates/(.*)$' => sprintf( 'index.php?plugin-customizer-template=%s',$wp_rewrite->preg_index( 1 ) ),
	 );

	 $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
	 return $wp_rewrite->rules;
}

save_settings.php

<?php
$customize_register_priority = 9;
add_action( 'wp_loaded', function() {
	add_action( 'customize_register', array( $this, 'customizer_plugin_settings' ) );
}, $customize_register_priority );
/**
 * Bailout if not called from the plugin menu links.
 */
if ( ! isset( $_GET[ PLUGIN_CUSTOMIZER_PARAM_NAME ] ) || PLUGIN_CUSTOMIZER_PARAM_VALUE !== wp_unslash( $_GET[ PLUGIN_CUSTOMIZER_PARAM_NAME ] ) ) {
	return;
}
add_action( 'wp_loaded', function() {
	add_action( 'customize_register', array( $this, 'customizer_plugin_sections' ) );
	add_action( 'customize_register', array( $this, 'customizer_plugin_controls' ) );
}, $customize_register_priority );