Added extended "pluginsDir" functionality.

I think yiiext/smarty-renderer wants me to add plugins to $smartyDir/plugins.
However, I want to have a custom directory with smarty plugins, i.e., I want
to use my github project https://github.com/kazaak/yiismarty.git.  So, this
pull request:

+ preserves BlindGenius' change, which adds $smartyDir/plugins
+ explicitly adds the plugins directory of yiiext/smarty-renderer.  I do it
relative to dirname(__FILE__) in ESmartyViewRenderer.php, there may be a better
way.
+ extends ESmartyViewRender's pluginsDir to support an array.  In my case, I
only need one ("application.extensions.smartyPlugins", where my yiismarty
repo resides).  However, I can envision using other similarly created repos or using my base repo from kazaak/yiismarty with a custom plugins dir for particular yii application.

Really appreciate yiiext/smarty-renderer!