Add command accessors to CommandGroupBuilder

[9999years]

Hello! I'm implementing command-error, a crate that provides nicer error messages for commands, like this:

`sh` failed: exit status: 1
Command failed: `sh -c 'echo puppy; false'`
Stdout:
  puppy

I'd like to add optional support for the command-group crate (9999years/command-error#2). Because traits can only be implemented once per type (and I think a generic trait would have poor ergonomics here), I'm attempting to impl command_error::CommandExt for command_group::builder::CommandGroupBuilder<'_, std::process::Command>, but it's not possible to access the underlying Command (to produce the name of the program that was run for error messages) with the current API.

Therefore, this PR adds getters to access the Command field of CommandGroupBuilder.

[passcod]

That's neat, but could you instead work on top of https://github.com/watchexec/process-wrap ? Development has moved there and I'm going to be freezing command-group; process-wrap gives you direct access to the command as part of its architecture.

[9999years]

Sure, I guess. Could you add a deprecation notice to this repo and the documentation?