Cronberry

- Updated examples in example script and README
- Updated docstrings for correctness

- Adds list command
- Fixes issue with writing shorthand syntax via `__str__()`
- Transforms Python exceptions to click.ClickException for CLI
- Removes unused sphinx configuration code

- 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.