Add Attribute and AttributeModifier API.

Thanks to __0x277F and Meeh on #spigot-dev for implementation advice.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/attribute/Attributable.java

@@ -0,0 +1,16 @@
+package org.bukkit.attribute;
+
+/**
+ * Represents an object which may contain attributes.
+ */
+public interface Attributable {
+
+    /**
+     * Gets the specified attribute instance from the object. This instance will
+     * be backed directly to the object and any changes will be visible at once.
+     *
+     * @param attribute the attribute to get
+     * @return the attribute instance or null if not applicable to this object
+     */
+    AttributeInstance getAttribute(Attribute attribute);
+}