Support --config with Config File

[oliver-pham]

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
Suggestions:

• 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.