Lootables input accepts lowercase entries (#32)

Make it so the lootables AnvilGUI can take in lowercase names for items.

Core/src/main/java/com/songoda/core/lootables/gui/GuiLootableEditor.java

@@ -45,7 +45,7 @@ public class GuiLootableEditor extends Gui {
                     gui.setAction((event1 -> {
                         try {
                             lootable.registerLoot(new LootBuilder().setMaterial(CompatibleMaterial
-                                    .valueOf(gui.getInputText().trim())).build());
+                                    .valueOf(gui.getInputText().trim().toUpperCase())).build());
                         } catch (IllegalArgumentException ex) {
                             event.player.sendMessage("That is not a valid material.");
                         }