Fix completion of long option values in Zsh

[segevfiner]

Long options take their argument either in the next word or after an equals sign, but the Zsh completion specified that they take it either in the same word with no separator or in the next word. See the documentation of the Zsh _arguments function for more information.

Sample for testing

extern crate clap;

use std::io;
use clap::{App, Arg, Shell};

fn build_cli() -> App<'static, 'static> {
    App::new("clap-test")
        .arg(Arg::with_name("wat")
            .long("wat")
            .takes_value(true)
            .possible_values(&["foo", "bar"]))
        .arg(Arg::with_name("completions")
            .long("completions"))
}

fn main() {
    let matches = build_cli().get_matches();

    if matches.is_present("completions") {
        build_cli().gen_completions_to("clap-test", Shell::Zsh, &mut io::stdout())
    }

    if let Some(wat) = matches.value_of("wat") {
        println!("{}", wat);
    }
}

Related #850

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This change is 

[mention-bot]

@segevfiner, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kbknapp to be a potential reviewer.

[kbknapp]

Thanks, ZSH completions is definitely an area where my knowledge is lacking and I was learning on the fly. I appreciate this!