v2: a huge restructurization

I know, I know, small commits all the way... But sometimes it is so hard to
stop the flow.

Huge changes.
Tried to modularize almost everything so it actually build a lot of
simple docset, Rust being just one of engines.

Added pyinvoke script to make it easier to use.

Things to improve: logging, docs, docset settings should need no coding
at all (json/toml/yaml)