Add interconversion with json

[moisutsu]

Add the methods to_json and from_json to the argument instance for interconversion with json.

to_json

from classopt import classopt

@classopt
class Opt:
    arg_int: int

opt = Opt.from_args()
print(opt.to_json())

opt.to_json("sample.json")

$ python sample.py 3
{"arg_int": 3}

$ cat sample.json
{"arg_int": 3}

to_dict

from classopt import classopt

@classopt
class Opt:
    arg_int: int

args_json = '{"arg_int": 3}'

opt = Opt.from_json(args_json)
print(opt)

opt = Opt.from_json("sample.json")
print(opt)

$ cat sample.json
{"arg_int": 3}

$ python sample.py 3
Opt(arg_int=3)
Opt(arg_int=3)