Add new feature: Support --config with config file

[hlavu]

Hi Dustin, I would like to add a new option -c or --config to your repo. This allows users to specify a config file storing options' values.
By doing so, a user could run the SSG in 2 ways:

#1:
ssg --input ./site --output ./build --stylesheet https://cdn.jsdelivr.net/npm/water.css@2/out/water.css --lang fr

#2:
ssg --config ./ssg-config.json

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

#Tasks:

• The -c or --config flags accept a file path to a JSON config file.
• If the file is missing, or can't be parsed as JSON, exit with an appropriate error message.
• If the -c or --config option is provided, ignore all other options (i.e., a config file overrides other options on the command line).
• The program should ignore any options in the config file it doesn't recognize. For example, if the SSG doesn't support stylesheets, ignore a stylesheet property.
• If the config file is missing any options assume the usual defaults. For example, use dist/ as the output directory if it isn't specified.
• Update README.md

[tuanthanh2067]

That sounds great, go for it!

[tuanthanh2067]

Closed by #22