Issue-11: Adding a new feature for the tool to support --config with config file

[nguyenhung15913]

Hi Jun Song, I am interested in your project. I would like to add a new feature to your jun-ssg so that it now can read the config file.
Details:

• Instead of passing arguments every time, users now can specify all of their options in a single config JSON file.
  Example, This CLI:
  jun-ssg -i hello.txt --s "example.com"
  Can be replaced with:
  jun-ssg --c config.json
  Where the config.json contains all of their options:

{
      "input": "hello.txt",
      "stylesheet": "example.com"
}

If you consider my request, please let me now thanks.