Add custom GFArgumentParser

[m4rc1e]

@simoncozens I've been toying with the idea of having our own ArgumentParser class. By default, it'll have the --show-tracebacks and --log-level commands set to sensible defaults. Once fully implemented, I could replace all ArgumentParsers with our one quite easily.

Still WIP since this may be a footgun idea.

[simoncozens]

Yes, like it!