Support --config with Config File

[Qiwen-Yu]

Description

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 either of the following:

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

# Option 2: use a config file
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.

Requirements

1. The -c or --config flags accept a file path to a JSON config file.
2. If the file is missing, or can't be parsed as JSON, exit with an appropriate error message.
3. If the -c or --config option is provided, ignore all other options (i.e., a config file overrides other options on the command line).
4. 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.
5. 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.