Improve addPlugin to not append default plugins to the list

lib/modifiers/config/addPlugin.js

@@ -1,16 +1,22 @@
 var PluginDependency = require('../../models/pluginDependency');
+var togglePlugin = require('./togglePlugin');
+var isDefaultPlugin = require('./isDefaultPlugin');
 
 /**
  * Add a plugin to a book's configuration
  * @param {Config} config
- * @param {String} plugin
+ * @param {String} pluginName
  * @param {String} version (optional)
  * @return {Config}
  */
-function addPlugin(config, plugin, version) {
-    var deps = config.getPluginDependencies();
+function addPlugin(config, pluginName, version) {
+    // For default plugin, we only ensure it is enabled
+    if (isDefaultPlugin(pluginName, version)) {
+        return togglePlugin(config, pluginName, true);
+    }
 
-    var dep = PluginDependency.create(plugin, version);
+    var deps = config.getPluginDependencies();
+    var dep = PluginDependency.create(pluginName, version);
 
     deps = deps.push(dep);
     return config.setPluginDependencies(deps);

lib/modifiers/config/isDefaultPlugin.js

@@ -0,0 +1,15 @@
+var DEFAULT_PLUGINS = require('../../constants/defaultPlugins');
+
+/**
+ * Test if a plugin is a default one
+ * @param {String} plugin
+ * @param {String} version
+ * @return {Boolean}
+ */
+function isDefaultPlugin(pluginName, version) {
+    return !!DEFAULT_PLUGINS.find(function(dep) {
+        return dep.getName() === pluginName && (!version || dep.getVersion() === version);
+    });
+}
+
+module.exports = isDefaultPlugin;

lib/modifiers/config/removePlugin.js

@@ -1,5 +1,5 @@
-var DEFAULT_PLUGINS = require('../../constants/defaultPlugins');
 var togglePlugin = require('./togglePlugin');
+var isDefaultPlugin = require('./isDefaultPlugin');
 
 /**
  * Remove a plugin from a book's configuration
@@ -10,12 +10,8 @@ var togglePlugin = require('./togglePlugin');
 function removePlugin(config, pluginName) {
     var deps = config.getPluginDependencies();
 
-    var isDefault = DEFAULT_PLUGINS.find(function(dep) {
-        return dep.getName() === pluginName;
-    });
-
     // For default plugin, we have to disable it instead of removing from the list
-    if (isDefault) {
+    if (isDefaultPlugin(pluginName)) {
         return togglePlugin(config, pluginName, false);
     }