Add ability to indent velocity content in templates

Would be nice to be able to filter velocity content from VM files, similarly to what we do when using the {{velocity}} macro. This would allow use to indent velocity scripts.

For example:

#macro(panelhiddenheader $title)
  #panelheader($title, false, true)
#end 

Instead of:

#macro(panelhiddenheader $title)
#panelheader($title, false, true)
#end 

(note that right now if you indent you'll get some extra "<p> </p>")