Make TemplateEngine configurable

Provides a get/set method for TemplateEngine with the default as GStringTemplateEngine. If needed a new implementation of TemplateEngine can be provided by using docxTemplater.setTemplateEngine(new CustomTemplateEngine())

GStringTemplateEngine works fine in most cases, but I had issues when I used it in a managed secure environment. I was getting exceptions at the parseClass calls inside the TemplatingEngine.

Also, this makes it more configurable.