Add support for more template engines

[jonbullock]

Add support for more template engines such as Velocity, Mustache.java, StringTemplate etc.

[Vad1mo]

I would also take a look at Thymeleaf specially in conjunction with Thymol or Thymus http://www.thymeleaf.org/ecosystem.html The JS tools allow to include fragments in offline mode. The result of this are templates visible in a browser. Quite handy for an offline tool like jbake.

[jonbullock]

Thanks for the suggestion, I'll definitely investigate Thymeleaf as another potential option.

[melix]

See #91