template_types: task 3: make a template engine instance registry on top of the existing template class registry

[chfw]

so that, custom template instance and ad-hoc could be created on demand during one moban command session.

current implementation does not expose the template engine instance, hence it is restrictive to leverage the full potential of the underlying templating engine.

in other words, moban's mission is to make itself as a hollow reed so that individual templating engines are not hindered by moban to show its full potential, and while being a moban affiliated templating engine, they are used in new front-tiers unknown to its creator. For example, jinja2 was used in Ansible, which is outside web development, although this is not attributed to moban but moban org wants to copy-cat and boost it.

#202