Add support for saved items.

Introduces the concept of a _saved item_; an in-game item that has been
captured in a YAML file via Bukkit's item serialization mechanism. These
items can be referenced in the config-file in all places that any other
normal item can be used, assuming the ThingManager is in charge of the
parsing. This should help bridge the gap between class chests and the
config-file by allowing any Bukkit serializable item stack to be stored
and referenced as if MobArena's item syntax directly supported it.

Three new setup commands are introduced to help manage the items, such
that they can be created, deleted, and loaded (for "editing" purposes).
The commands are somewhat rough around the edges and may need a little
bit of polish going forward.

Together with the new inventory referencing Things, this functionality
should help provide most of the flexibility people have been missing
from the item syntax for about a decade... Hell, it's about time.

Closes #212

changelog.md

@@ -13,6 +13,7 @@ These changes will (most likely) be included in the next version.
 ## [Unreleased]
 ### Added
 - Support for chest references in item syntax. The new `inv` syntax allows for referencing container indices in the config-file. This should help bridge the gap between class chests and various other parts of the config-file, such as rewards and upgrade waves.
+- Support for saved items. The new `/ma save-item` command can be used to save the currently held item to disk, which allows it to be used in various places in the config-file. This should help bridge the gap between the config-file and class chests for config-file centric setups.
 
 ### Fixed
 - Explosion damage caused by Exploding Sheep now correctly counts as monster damage. This means that the explosions only affect other mobs if the per-arena setting `monster-infight` is set to `true`.