rust-chg: add callback to handle pager and shell command requests This could be inlined into the ChgRunCommand state to be introduced by the next patch, but it seemed good to separate any user interactions from the IPC code.

// Copyright 2018 Yuya Nishihara <yuya@tcha.org>
//
// This software may be used and distributed according to the terms of the
// GNU General Public License version 2 or any later version.

fn main() {
}