Add argfile support

[XAMPPRocky]

Maintainer's notes:

• Prior art:
  • javac's documentation for accepting @
  • argparse's fromfile_prefix_chars
  • description of rg
  • Microsoft Response Files

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Describe your use case

I've received requests (XAMPPRocky/tokei#437) to add argfile support to tokei, it would be nice if this was provided in clap so that it can be shared and reused by all crates.

Argfiles aren't a specific standard, and there are varying levels of support. The best documentation I found on it was documentation on javac's CLI.

Describe the solution you'd like

CLI

tokei @argfile

Rust

App::new("My Super Program").argfile(true)

[CreepySkeleton]

Here's a formal description for the sake of possible implementors:

• App::new("My Super Program").global_setting(AppSettings::AllowArgfiles) tells clap to look after positional arguments starting with @. These args will be treated as @argfiles.
• @argfiles can only be preceding so explicit cli options override them, i.e it's not possible to do something like app -J --foo @argfile1 bar @argfile2 baz. This fact should simplify the implementation considerably.
• Each @argfile is treated as a list of command line options/args. In fact, the args simply inserted into processing queue in place of @argfile occurrence.
  • One line corresponds to one arg. The following @argfile corresponds to three args.

  -J foo
  Bar
  "rm -rf /"
  

  • Each line is treated verbatim. Nothing is stripped, no escapes.
  • If the file end with a newline character it will be stripped (but only one!).
  • If a file is not found - it's a parsing error.

[pksunkara]

So, with #1697, I rewrote how input is being parsed. That would enable us to use @argfiles even in the middle.

[xadaemon]

@pksunkara are you working on this or can I give it a go?

[pksunkara]

Please go ahead.

[xadaemon]

I will implement to @CreepySkeleton's spec then.

[pksunkara]

If the App is taking an argfile method, then it's better if it's implemented for all subcommands instead of just at the end.