Merge: lib/config: better tool configuration

This PR reworks some aspects of the `lib/config`:
* Add option `-?` as help
* Introduce `--stub-man` option (almost the same thing than in `ToolContext`)
* Make `nitmd` use `config` instead of `opts`
* Define a better `tool_description` for `github/loader`

Pull-Request: #2674
Reviewed-by: Jean Privat <jean@pryen.org>