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, arguments passed to handling function will be `str`
- `guides.literal` is default guide for any type-hint except for `str`
Rename `Hue` to `Palette`
- Rename `Hue` to `Palette`
- Add `BOLD` formatting option