Require one argument for optional parameters

[josebolos]

When an optional parameter like -p that requires an argument is used but an argument is not passed, this will cause the app to fail as expected instead of using a random value. For example, port was being set to 3000 instead of the default 5401.

This hopefully addresses #145.

[jpw]

Tests plz, soz.

[josebolos]

@jpw I'm unsure of what needs to be tested here. The processing of the arguments is done by yargs, which in theory we should trust as being already proven to be working by its authors, and in fact that's the case for every other command line argument that we accept.

If we want to start testing that all the CLI arguments are working as expected, then we need to find something that allows us to test that. I don't even know what that would be, it's definitely not unit testing. Something that could start shunter and finding a way of outputting the results of all the CLI arguments, which shunter doesn't even have at the moment. So that would be a new feature to be added to shunter.

Is this worth it, considering the scope of this PR? I'm not aware of any other of our apps running these kind of tests.

[jpw]

Thanks for having a look! Similar tests can be seen in https://github.com/springernature/shunter/blob/master/tests/server/core/worker.js ;)

While adding nargs params looks like a Good Thing, does it actually fix the original issue?