Add method editPlugin to config modifiers

lib/modifiers/config/editPlugin.js

@@ -0,0 +1,16 @@
+
+/**
+ * Edit ocnfiguration of a plugin
+ * @param {Book} book
+ * @param {String} plugin
+ * @param {Object} pluginConfig
+ * @return {Book}
+ */
+function editPlugin(book, pluginName, pluginConfig) {
+    var config = book.getConfig();
+    config = config.set('pluginsConfig.'+pluginName, pluginConfig);
+
+    return book.setConfig(config);
+}
+
+module.exports = editPlugin;

lib/modifiers/config/index.js

@@ -2,5 +2,6 @@
 module.exports = {
     addPlugin:    require('./addPlugin'),
     removePlugin: require('./removePlugin'),
-    togglePlugin: require('./togglePlugin')
+    togglePlugin: require('./togglePlugin'),
+    editPlugin:   require('./editPlugin')
 };