title: issue getting dynamic Config parameter in Grails taglib
body: <p>I have a dynamic config parameter I want to get like:</p>
  
  <pre><code>    String srcProperty = "${attrs ['src']}.audio" + ((attrs['locale'])? "_${attrs['locale']}" : '')
  
      assert srcProperty == "prompt.welcomeMessageOverrideGreeting.audio"
  </code></pre>
  
  <p>where my config has:</p>
  
  <pre><code>prompt{
      welcomeMessageOverrideGreeting {
          audio = "/en/someFileName.wav"
          txt = "Text alternative for /en/someFileName.wav"
          audio_es = "/es/promptFileName.wav"
          txt_es = "Texto alternativo para /es/someFileName.wav"
      }
  }
  </code></pre>
  
  <p>While this works fine:</p>
  
  <pre><code>String audio = "${config.prompt.welcomeMessageOverrideGreeting.audio}"
  </code></pre>
  
  <p>and:
          assert "${config.prompt.welcomeMessageOverrideGreeting.audio}" == "/en/someFileName.wav"</p>
  
  <p>I can not get this to work:</p>
  
  <pre><code>String audio = config.getProperty("prompt.welcomeMessageOverrideGreeting.audio")
  </code></pre>