Keywords should be available in Jekyll front matter

[dagoss]

When keywords (or indexterms) appear in a topic, they should be added to the Jekyll frontmatter as tags. So this:

<prolog>
 <metadata>
  <keywords>
   <keyword>installing</keyword>
   <keyword>uninstalling</keyword>
   <keyword>prerequisites</keyword> 
   <keyword>helps</keyword>
   <keyword>wizards</keyword>
  </keywords>
 </metadata>
</prolog>

Should result in this

---
tags: instaling, uninstalling, prerequisites, helps, wizards

---

It should be possible to turn this on/off with an Ant parameter