Add new SSG Feature: Support --config with Config File

[MizuhoOkimoto]

This is fixes #14
I've added a new feature that supports a config file, so if users want to be able to specify all of their SSG options in a JSON formatted configuration file instead of having to pass them all as command-line arguments every time.
Specifically, I added yargs option which is -c, --config, and readFileSync function on the textToHTML.js file.
Also, I added the following config file:
./ssg-config.json

{
    "input": "./site",
    "output": "./build",
    "stylesheet": "https://cdn.jsdelivr.net/npm/water.css@2/out/water.css",
    "lang": "fr"
}

Please let me know if you want any changes or anything I can improve.
Thank you for considering.

[AndreWillomitzer]

Please edit the readME file with details about config support and examples showing how to use it.

[MizuhoOkimoto]

I added 2 lines to README.md file based on the new features which are the Example and Features sections.
Please review it and let me know if I need to improve.