Cliar

- Make `_root` not an abstract method.

- Fix critical bug that disallowed string params.

- Boolean handler params are converted into `store_true` arguments. Before that, params with default value of `True` were much confusingly converted into `store_false` arguments.
- Support `List[int]` and similar arg types. If the param type is a subclass of `typing.Iterable` and has a type specified in brackets, it's converted into multivalue arg of the type in the brackets.
- Do not print help whenever `_root` command is invoked.
- Convert the `cliar` module into a package.
- Add tests.
- Switch to Poetry.

- Add the ability to set help messages for arguments.
- Add the ability to set metavars for arguments.

- **[Breaks backward compatibility]** Base CLI class renamed from `CLI` to `Cliar`.
- Fixed a bug where commandline args with dashes weren't mapped to corresponding param names with underscores.

- Add the ability to override mapping between commandline args and and handler params. By default, handler params correspond to args of the same name with underscores replaced with dashes.