Use OS-native argument handling

[HeroicKatora]

This fixes edge cases where the output file might have a non-UTF8
filename, this would panic when using env::args. It also uses the
standard file_name method instead of an ad-hoc parsers for retrieving
the program's command name in its help.

[HeroicKatora]

This doesn't actually fix everything. What I was hoping to achieve was to handle arguments such as:

#!/bin/bash
echo hello > $'./\xff' # Okay, a valid file under Linux/ext4
echo hello | rewrite $'./\xff' # Currently:
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "\xFF"', library/std/src/env.rs:772:51
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Instead, it now fails in getopts, more specifically here where it will try to match the argument and results in returning an error:

Unrecognized option: '"./\xFF"'
Usage: rewrite FILE [options]

Options:
    -h, --help          prints this help info

[mqudsi]

Hello @HeroicKatora and thanks for the PR. I'll take a look and see why getopts doesn't like this.

[HeroicKatora]

I found a tracking issue: rust-lang/getopts#97

[mqudsi]

@HeroicKatora can you please try building from master with 2367be0?

[HeroicKatora]

Works like a charm 👍

[mqudsi]

Awesome, thanks for confirming!

[HeroicKatora]

Could you do a cargo release for this feature? Maybe consider even 1.0 as the interface itself looks quite stable now, unless you have some concrete plans for changing it soon.

[mqudsi]

Done! You've been named in the release notes.