Allow for a custom config file to be specified

Allow for a custom config file to be specified
Optimist has been deprecated for a while now. It's probably better to use something like [yargs](https://github.com/chevex/yargs). I know its documentation looks like it's been written by a 7-year old retard, but it works pretty well.

See https://github.com/fronteerio/grasshopper-scripts/blob/master/old-stack-interaction/orgunit-tree.js#L26 for an example