New SSG Feature: Support --config with JSON config file

[a-parris21]

Hello,
I would like to add a new feature to SSGifier. Through this new feature, it would be able to support the Config file with the
--config flag.

Here is the list of the changes I would implement, as described in the Lab 4 instructions:

• 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.

I will reach out to you via Slack.

[humphd]

Looks like this didn't closed when #9 was merged.