Pipen-args

- ♻️ Refactor to adopt pipen 0.4
- ✅ Add tests

**Full Changelog**: https://github.com/pwwang/pipen-args/compare/0.3.5...0.4.0

- 🐛 Fix `flatten_proc_args` auto turns to `True` when `len(procs) > 1`

**Full Changelog**: https://github.com/pwwang/pipen-args/compare/0.3.4...0.3.5

- 🐛 Fix `outdir` not working when `name` is given

**Full Changelog**: https://github.com/pwwang/pipen-args/compare/0.3.3...0.3.4

- ⬆️ Pump pipen to 0.3.8
-🚨 Fix linting for examples
- ⬆️ Bump pardoc to 0.1

**Full Changelog**: https://github.com/pwwang/pipen-args/compare/0.3.2...0.3.3

- 🐛 Fix wrong configs for single-proc pipeline
- ✨ Allow input to be scalar in config file

**Full Changelog**: https://github.com/pwwang/pipen-args/compare/0.3.1...0.3.2

- 📝 Add example for `from pipen_args import config`
- ✨ Support `--cache force` from command line (only when it's not defined in config file)

> NOTE: When we have `[cache]` defined in config file, only the same type of
> value is accepted from the command line argument `--cache`. But cache supports
> `True`, `False` and `"force"`. So when you need `"--cache force"` from command
> line, you have to remove it from the config file if it defaults to a bool.


**Full Changelog**: https://github.com/pwwang/pipen-args/compare/0.3.0...0.3.1