Support --config with Config File

[DukeManh]

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. For example, consider 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"
}

A user could run the SSG by doing the following:

ssg --config ./ssg-config.json

The config file option means we can have a much shorter command, and instead store our options in a file.

[tpmai22]

Hi Duke,
It is my honor to get your brilliant help on my project. I would love to do the same thing on your repo if you allowed me to ?

[DukeManh]

Please go ahead. Thank you James.