Drop Decodable dependency for call_main_without_stdin

[dtolnay]

pub fn call_main_without_stdin<Flags: Decodable>(
            exec: fn(Flags, &Config) -> CliResult,
            config: &Config,
            usage: &str,
            args: &[String],
            options_first: bool) -> CliResult

This may be blocked on docopt/docopt.rs#128 unless we replace docopt.

[alexcrichton]

👍

[alexcrichton]

(note that this is indeed blocked on docopt)