Add inventory, and the ability to pick up items

Add inventory as a basic vector of items attached to the character, and
the ability to pick up a single item where the character stands

src/messages.toml

@@ -2,6 +2,7 @@
 welcome = "Welcome to Xanthous, {{character.name}}! It's dangerous out there, why not stay inside?"
 describe_entities = "You see here {{descriptions}}"
 describe_no_entities = "You see nothing here."
+pick_up = "You pick up the {{item.name}}."
 
 [combat]
 attack = "You attack the {{creature.name}}."