Feature: able to parse Optional[Type] field to None from command-line

[anqixu]

Currently, if a field with type-hint Optional[Type] has a non-optional default value, there's no way to set it to None by parsing from the command line. E.g.

@dataclass
class Config2:
    var_with_default: Optional[int] = 42

$ ./run.py --var_with_default None

This PR adds this feature, specifically by parsing the string literal "None" into None for optional-typed fields.

[anqixu]

@lebrice could you kindly have a review of the updated PR? Previous version failed CI due to silly issue (comparing type() directly), which I since fixed.