Fix minor README mistakes.

Update README with information about CLI and inline command execution.

Change development status to `4 - Beta`

Rename `ValidationError` so name better reflects its purpose.

Extract guides out of type-hints.

- Rename `validators` to `guides`
- Extract guides out of type-hints.
- If for given command no guides was explicitly defined by user use
`guides.literal` as default guide for any type-hint except for `str`.

Prepare package versioning

* Use setuptools_scm for package versioning.
* Add Makefile with devel chores