ArgParser rewrite

[YoshiRulz]

I'd still like to improve the documentation further, but this is functional already. If you have scripts for e.g. encoding please test them.

[Morilli]

If we do this, is there a way to get rid of the ParsedCLIFlags struct as well? Feels like unnecessary duplication.

[YoshiRulz]

If we do this, is there a way to get rid of the ParsedCLIFlags struct as well? Feels like unnecessary duplication.

Yes, by exposing the Options, returning the ParseResult, and moving the GetValueForOption calls throughout MainForm. I will try to do that for this PR.

[YoshiRulz]

Should --help group the flags semantically? (Re-ordering the list is trivial, unfortunately adding headers isn't.) I chose to list them alphabetically since that's what I'm used to from Unix tools.