Add support for Thymeleaf template engine

This pull request adds support for the Thymeleaf template engine.

* template file names *must* end with .thyme
* ability to override the default HTML5 mode with template.<type>.thymeleaf.mode=<mode>
* does *NOT* support lazy loaded models due to Thymeleaf limitations

Some remarks:

* I have done this to demonstrate how to add support for a new template engine
* I don't think this should be used for sites with lots of pages. Thymeleaf is kind of limited when it comes to custom models, and as such, doesn't allow loading lazily. This implies that the *whole* database is loaded in memory for every page.
* I strongly suggest, if this PR is merged, that we start separating jbake-core from template engines/rendering engines by having submodules