Wait for pager process before exiting

[dandavison]

Fixes #463 Replaces #464

Hi @marcoieni, I think this it what we were looking for. It took me a while to find, but this calls wait() on the child pager process before exiting. I was just about to merge #464 but then I read these sentences (https://doc.rust-lang.org/std/process/fn.exit.html) which seemed like the clue we were looking for.

Function std::process::exit
Note that because this function never returns, and that it terminates the process, no destructors on the current stack or any other thread's stack will be run. If a clean shutdown is needed it is recommended to only call this function at a known point where there are no more destructors left to run.

[dandavison]

https://github.com/dandavison/delta/blob/master/src/bat_utils/output.rs#L154-L160

impl Drop for OutputType {
    fn drop(&mut self) {
        if let OutputType::Pager(ref mut command) = *self {
            let _ = command.wait();
        }
    }
}

[marcoieni]

Oh, so drop wasn't called. Nice, today I learned something, thanks!