Add a new --eval option, to be used in scripts

Add a new --eval option, to be used in scripts

Cosmetic changes for the new PyPI

New features:

- izk is now installable via docker
- allow configuration via environment variables, to ease docker usage

Improvements:
- Run flake8 checks as part of the CI
- Use pipenv to lock and install the development environment

Improvements:

- Enable vi mode by default
- Don't display a traceback when mashing Ctrl-D on exit confirmation message

Fixes:
- `raw` commands are now usable (reported by @hzluyang)

New command:

 - `edit`: opens an editor to edit the content of a znode

 Improvements:
 - If only a path is given to `set`, an editor will be opened to type-in the ZNode data
 - Command validation refactoring

Add missing dependency

Bump version to 0.2.0

Release version 0.1.1