Override toMap fromMap case style (snake_case or camelCase)

[pauldemarco]

camelCase (default):

Address.fromMap(Map<String, dynamic> m)
      : city = m['city'],
        postalCode = m['postalCode'],
        state = m['state'];

  Map<String, dynamic> toMap() => {
        'city': city,
        'postalCode': postalCode,
        'state': state
      };

snake_case:

Address.fromMap(Map<String, dynamic> m)
      : city = m['city'],
        postalCode = m['postal_code'],
        state = m['state'];

  Map<String, dynamic> toMap() => {
        'city': city,
        'postal_code': postalCode,
        'state': state
      };

User should be able to set this in build config file, or per class like:

@data(caseStyle=CaseStyle.snake)