Support --config with JSON Config File

[menghif]

Hi, I would like add a --config option to specify all of the other SSG options in a JSON formatted configuration file.

Example:

node index.js --config options.json

where options.json is equal to:

{
  "input": "./docs",
  "output": "./web"
}

would be the equivalent of doing node index.js -i ./docs -o ./web