Expose Slave to instance plugins

Allow the slave object to be accessed by instance plugins through the
this.slave property.

lib/plugin.js

@@ -1,3 +1,7 @@
+/**
+ * Plugin interfaces and utilities.
+ * @module
+ */
 "use strict";
 const fs = require("fs-extra");
 const path = require("path");
@@ -9,9 +13,27 @@ const errors = require("lib/errors");
  * Base class for instance plugins
  */
 class BaseInstancePlugin {
-	constructor(info, instance) {
+	constructor(info, instance, slave) {
+		/**
+		 * The plugin's own info module
+		 */
 		this.info = info;
+
+		/**
+		 * Instance the plugin started for
+		 * @type {Instance}
+		 */
 		this.instance = instance;
+
+		/**
+		 * Slave running the instance
+		 *
+		 * With the exepction of accessing the slave's config you should
+		 * avoid ineracting with the slave object directly.
+		 *
+		 * @type {Slave}
+		 */
+		this.slave = slave;
 	}
 
 	/**