原文：
Plugin Initialization and Lifetime
One instance of a plugin object is created for the life of each WebView. Plugins are not instantiated until they are first referenced by a call from JavaScript, unlesswith an onload name attribute is set to “true” in config.xml. For example,
<feature name="Echo">
<param name="android-package" value="<full_name_including_namespace>" />
<param name="onload" value="true" />
</feature>
如果不加，则插件只有在被首次调用时才会被实例化
