SSG update - new feture (Support --config with Config File)

[jjung99]

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.

[jjung99]

Please check the pull request! thanks :) created pull request