Improvements to document templates

[jmiranda]

I've been meaning to update our very outdated Docx4j document template implementation. One thing that has always bothered me is that we had a very basic (homegrown) template syntax that didn't really add much value and made the document templates less expressive.

I just found an awesome library that allows the use of Velocity or Freemarker as template language: https://github.com/opensagres/xdocreport