Add PLUS and MINUS to list of keyboard keys

Author: victorb

src/input/Keyboard.js

@@ -656,3 +656,5 @@ Phaser.Keyboard.INSERT = 45;
 Phaser.Keyboard.DELETE = 46;
 Phaser.Keyboard.HELP = 47;
 Phaser.Keyboard.NUM_LOCK = 144;
+Phaser.Keyboard.PLUS = 43;
+Phaser.Keyboard.MINUS = 45;