Cronberry

- Adds the use of `MAILTO`, `MAILFROM`, `PATH`, `SHELL`, and `CRON_TZ` environment variables for each job
- Changes the `job` command to `view` to make it more distinct from the `jobs` command
- `cronberry.parse_crontab()` now returns an empty list if no jobs are found

Added the `enter` command, which can add jobs by title and command from the command line.

Updated pyproject.toml to show README and fix other small mistakes.

API updated for consistency, additional infrastructure added.

Initial release of `cronberry`!