Create wp_bstrp_nav_menu.php

Icon font input proposal for WP-Bootstrap-Navwalker. 
Add this file in theme directory and 'include_once( 'wp_bstrp_nav_menu.php' );' 
before wp-bootstrap-navwalker line in 'functions.php'. 
An icon font field will appear in the Menu Item settings page. 
TODO input validation/sanitization, code enhancements