Add u_i initialization and craft registration callbacks (#184)

Main purpose: custom recipe sorting provided by cuszom mods.

doc/mod_api.txt

@@ -20,6 +20,64 @@ Grouped by use-case, afterwards sorted alphabetically.
 	* Checks whether creative is enabled or the player has `creative`
 
 
+Callbacks
+---------
+
+Register a callback that will be run whenever a craft is registered via unified_inventory.register_craft:
+
+	unified_inventory.register_on_craft_registered(
+		function (item_name, options)
+			-- item_name (string): name of the output item, equivalent to `ItemStack:get_name()`
+			-- options (table): definition table of crafts registered by `unified_inventory.register_craft`
+		end
+	)
+
+Register a callback that will be run after all mods have loaded and after the unified_inventory mod has initialised all its internal structures:
+
+	unified_inventory.register_on_initialized(callback)
+		-- The callback is passed no arguments
+
+
+Accessing Data
+--------------
+
+These methods should be used instead of accessing the unified_inventory data structures directly - this will ensure your code survives any potential restructuring of the mod.
+
+Get a list of recipes for a particular output item:
+
+	unified_inventory.get_recipe_list(output_item)
+
+	Returns a list of tables, each holding a recipe definition, like:
+	{
+		{
+			type = "normal",
+			items = { "default:stick", "default:stick", "default:stick", "default:stick" },
+			output = "default:wood",
+			width = 2
+		},
+		{
+			type = "shapeless",
+			items = { "default:tree" },
+			output = "default:wood 4",
+			width = 0
+		},
+		...
+	}
+
+Get a list of all the output items crafts have been registered for:
+
+	unified_inventory.get_registered_outputs()
+
+	Returns a list of item names, like:
+	{
+		"default:stone",
+		"default:chest",
+		"default:brick",
+		"doors:door_wood",
+		...
+	}
+
+
 Pages
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