Parse an explicit config file to set global options

[kumar303]

This is a stepping stone toward implementing all of #176

For this step, let's do the following:

• Add a new global --config, -c option that accepts a file path
• Parse the config file with require() as if it were a NodeJS module
• The configured value for any option can be overridden by a specific command line value

See #738 for handling default values