Update update tool to recognize custom icons.json

tool/update.sh

 #!/usr/bin/env bash
+if [ -e ./icons.json ];
+then 
+echo "Custom icons.json found, using local data only."
 
+dart ./tool/generate_font.dart ./icons.json
+dart ./tool/generate_example.dart ./icons.json
+dartfmt -w ./lib/font_awesome_flutter.dart
+dartfmt -w ./example/lib/icons.dart
+
+else
+echo "Updating icons to newest version."
 pushd lib/fonts/
 
 curl -O -L "https://raw.githubusercontent.com/FortAwesome/Font-Awesome/master/webfonts/fa-brands-400.ttf"
@@ -16,3 +26,4 @@ dartfmt -w ./lib/font_awesome_flutter.dart
 dartfmt -w ./example/lib/icons.dart
 
 rm /tmp/icons.json
+fi