**BREAKING CHANGE:** included config now loads envfiles relative to the cwd
specified on the include option (if any).

New/updated special env vars:
- POE_CONF_DIR: the path to the dir containing the config file that defines the
  running task or the cwd set when including the config
- POE_DEBUG makes poe print more stuff to help with debugging

Refactor specifics:
- Introduce PoeOptions class for deserializing config. I would have used
  pydantic but it's too heavy for use in a CLI tool like this.
- Keep config from different files separate within PoeConfig
- Collect config validation logic in the config classes and declare config types
  in more detail
- Make error reporting more structured and consistent

Also:
- Fixed bug that caused some task validation to be skipped for tasks with args
- Improve consistency and coverage for config validation
- Fix crash in switch with args naked string for control
- PoeConfig.load will load everything again if called a second time
- remove pylint from the project
- Enable ANSI color output by default inside GitHub Actions