Do option validation

Currently gluegun doesn't do any option validation so you can invoke with --any-random-option and it won't complain. Also it only works with options coming after the positional arguments, which is highly unconventional and unfriendly.

gluegun uses yargs under the hood but I can't figure out how to fix this. Based on a quick look at the gluegun source I think it's a fundamental limitation of gluegun. TBH I'm tempted to scrap gluegun because it doesn't seem to add much value to this project.