Add json.loads as new required_settings_type

[drgarcia1986]

Useful to override complex settings from env, f.ex:

• settings file

SIMPLE_CONF = {'simple': 'conf'}

SIMPLE_SETTINGS = {
    'OVERRIDE_BY_ENV': True,
    'REQUIRED_SETTINGS_TYPES': {
        'SIMPLE_CONF': 'json.loads',
    }
}

• app

from simple_settings import settings

print(settings.SIMPLE_CONF)

• execution

$ python app.py
{'simple': 'conf'}

$ SIMPLE_CONF='{"from": "env"}' python app.py
{'from': 'env'}

[coveralls]

Coverage remained the same at 100.0% when pulling ed22378 on required_settings_type_json into 1278bd6 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling ed22378 on required_settings_type_json into 1278bd6 on master.