argparse

Apparently GoDoc cannot show documentation for exported methods of un-exported structs, and it makes no sense to export parser and/or command, thus this project should provide its own documentation page.

Is your feature request related to a problem? Please describe.
'choices' does not seem to be checked in load() method and invalid value can be set via the method.

To reproduce:

config.yml

animal: bird

argtest.py

import dataclasses
import simple_parsing as sp
from simple_parsing.helpers import Serializable

@dataclasses.dataclass
class Config(Serializabl