A bunch of improvements in `expand` plugin:
* introduce decorators to tell to the engine what way it must process results
* introduce a way to process jinja2 templates
* (re)introdude a way to use TemplateInterface2 to edit generated snippet
* UDFs w/o any decorator is a fallback to a `legacy` way: just insert
generated text into a document instead of a name of expansion function ;)
* C++ expansion functions are rewritten using jinja2 templates and allowed
TI2 for postprocessing
* almost all C++ expansions get some new features espacially class generation
expander `cl`... One may try smth like cl(test,@c,!cc,mv)+<Ctrl+E> :)