feat: support --help and --version CLI arguments

[Aloso]

This adds --help/-h and --version/-V CLI arguments, following the defaults used by clap. When any of these options is found, we exit with status code 0, which indicates success. When an unknown argument is supplied, we exit with the status code 1. Arguments after --help/--version are ignored, which I think is not a big deal.

If the plan is to support more CLI arguments, it would make sense to switch to a proper argument parser like clap or lexopt.

Closes #264

[GyulyVGC]

I think it's better to not include clap just for these two arguments, as you did.
I originally thought to also implement #243 through CLI arguments, but in the end I think I'll implement it as a feature directly inside the GUI for ease of the users.

Seems fine to me, I will test it as soon as I have time to do so.
Just a little notice is that we could also include -v in lowercase (I've seen it more often lowercase wrt uppercase).

[Aloso]

@GyulyVGC sure, I'll add -v as well.

Regarding the geoip database, another option is to use an environment variable, e.g.

SNIFFNET_GEOIP_COUNTRY=/path/to/country.mmdb sniffnet

which can be added to a shell config file for convenience.

[GyulyVGC]

I'm already using confy to save configurations for the app (settings info and last successfully sniffed adapter) and I was thinking to adopt the same approach.

[GyulyVGC]

@all-contributors please add @Aloso for code.

[allcontributors]

@GyulyVGC

I've put up a pull request to add @Aloso! 🎉