Feature Request: JSON Support

[erikshafer]

In addition to natively taking in and parsing a YAML file, with .NET Core promoting use of files such as appsettings.json, it would be incredibly useful for this library to support JSON out of the box. This could be done via StreamReader, but having support like with YAML would follow the paradigm Microsoft has set.

[mattglet]

I would also like to see an overload where I can pass in my keys to the Load method. I prefer to keep these types of settings encrypted so that when they are committed to source control they are not in plain text (.NET Framework supports this via web.config encryption key containers). Having the YAML file requires that my credentials be stored and committed via plain text :(