Secrets-vault

- Breaking change: Change encryption backend to AES-256-GCM
- Add `--export` option to envify.
- Update docs

- Move common params before command

- Relax requirements constraints
- Minor CLI tool fixes

- Fix requirements not listed in package

- Add envify command
- Refactor CLI tool
- Breaking Python API changes: persist() has been renamed to save(), and init() has been renamed to create().

- Add del command