Enable to specify an alternate template engine

[bruno]

There is currently no way to change the template engine through a plugin, or even through a PHP constant in config.php.

To be able to allow people to test experimental features introduced by plugins, it would be nice to let plugins override the default template engine (if not possible, being able to do that through a constant in config.php), with their own engine.

That would enable to test experiments (with twig on public side for instance) for the versions to come, without having to install the unstable version.