Add provider for Flag representing its values by names

[zhPavel]

class Color(Flag):
    RED = auto()
    BLUE = auto()
    GREEN = auto()
    WHITE = RED | BLUE | GREEN

Color.RED -> ['RED'] Color.RED | Color.BLUE -> ['RED', 'BLUE']

The provider should have such options:

1. Interpret string as a list of one element or forbid this
2. Forbid duplicating values at list parsing
3. Use special names for compound values (e.g. Color.RED | Color.BLUE | GREEN -> ['WHITE'])