Dune executable should accept `--help` and `--version` arguments

[Aloso]

Typing dune --version should print version information. dune --help should print help information or open the man page.

Also, many shells accept a -c, --command argument to execute commands and print the result, e.g.

~ $ bash -c "echo hello world"
hello world
~ $

[Aloso]

@adam-mcdaniel I'd like to use clap 3.0.0-beta.5 for this.

[adam-mcdaniel]

Yes, I agree. I love clap a lot: I think it would be great for this. I can add it.