functions.php

<?php
add_filter( 'menu_title', 'arunasco_icons' );
function arunasco_title( $title ) {
  $title = do_shortcode( $title );
  return $title;
}

// [icon name="fa-coffee"]
add_shortcode( 'icon', 'arunasco_icons_shortcode' );
function arunasco_icons_shortcode( $attr=array(), $content = '' ) {
  $defaults = array(
    'name' => '',
  );
  $attr = wp_parse_args( $attr, $defaults );
  $result = "<i class='fa {$attr['name']}'></i>";
  return $result;
}