Move CLI to rune package so it can be re-used in other projects

[udoprog]

This allows downstream projects to build a CLI which has their own context configured.

This is accomplished by adding the cli feature to rune:

rune = { version = "0.12.3", path = "../rune", features = ["cli"] }

And setup a binary with the following:

// src/bin/my_cli.rs

fn main() {
    rune::cli::Entry::new()
        .about(format_args!("My Project"))
        .context(&mut |opts| {
            let mut c = rune::Context::with_config(opts.capture.is_none())?;
            c.install(my_project::module())?;
            Ok(c)
        })
        .run();
}

It's important to obey the opts.capture option, since it will be used when running tests.

Once this is complete, you can run your binary to say build documentation or run tests:

cargo run --bin my-cli -- doc
cargo run --bin my-cli -- test