Create an html/js output plugin

[avtobiff]

An html/js output plugin for Yanger similar to pyang jstree would be nice.

The plugin should emit HTML and Javascript to create an interactive
page where the schema nodes can be expanded and collapsed.

It would increase usability serving the generated files directly. E.g. something
like yanger -f jstree --jstree-server localhost:8000 model.yang
would serve the resulting page for model.yang at http://localhost:8000/.

Additionally having the possibility to browse referenced YANG models,
i.e. compile and serve them on the fly via hyperlinks would be neat.