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
Update wp_bootstrap_navwalker.php

Updated to support icon fonts input feature in the backend
Update wp_bstrp_nav_menu.php

Added wp_kses validation to wp_bstrp_custom_field_update, leaves database clean when empty 
-thank you JD Grimes!
Merge pull request #94 from cristovaov/dev

Icon font input proposal for WP-Bootstrap-Navwalker.