uutils: implement shell completions

[miDeb]

This adds a hidden completion subcommand to coreutils. When invoked with
coreutils completion <utility> <shell> a completion file will be printed
to stdout. When running make install those files will be created for all
utilities and copied to the appropriate locations.
make install will install completions for zsh, fish and bash; however,
clap also supports generating completions for powershell and elvish.

With this patch all utilities are required to have a publich uu_app function
that returns a clap::App in addition to the uumain function.

[tertsdiepraam]

Amazing! Nice solution for hashsum too! I just wondered whether usage and after_help should be in uu_app too. I also have some minor suggestions for some utils, which we can make issues for if you don't want to fix them here,

[tertsdiepraam]

Sorry, I introduced some conflicts this by first merging #2361, could you take a look at those? After that, we should merge this quickly, because it looks good and it will keep conflicting with other PR's.

[sylvestre]

Impressive work. It would be great to have a section in the README to document the user feature :)
thanks

[miDeb]

I'll fix the merge conflicts myself, if you don't mind :)

[sylvestre]

@miDeb oh, yes, please, I made a mistake :(
sorry

[sylvestre]

No big deal but maybe handle the error a bit better:

$ cargo run completion ls zs
    Finished dev [unoptimized + debuginfo] target(s) in 0.19s
     Running `target/debug/coreutils completion ls zs`
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "[valid values: bash, fish, zsh, powershell, elvish]"', src/bin/coreutils.rs:148:24
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[sylvestre]

or

$ cargo run completion ls
    Finished dev [unoptimized + debuginfo] target(s) in 0.19s
     Running `target/debug/coreutils completion ls`
thread 'main' panicked at 'expected shell as the second parameter', src/bin/coreutils.rs:134:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace