shared_dirs_improvements: Final cleanup before v1 (#50)

This patch consists of:
- code cleanups,
- readme/docs cleanups,
- a simplification of the `command`/`expect` feature,
- a bump of the tag from `v0` to `v1` in the readme.

Signed-off-by: Krzysztof Bieganski <kbieganski@antmicro.com>