feat: add getters to mirror those available on std::process::Command

[mcky]

As discussed in #212, this adds the following getters that dispatch to the underlying std::process::Command

pub fn get_program(&self) -> &ffi::OsStr {}
pub fn get_args(&self) -> process::CommandArgs<'_> {}
pub fn get_envs<'a>(&'a self) -> process::CommandEnvs<'a> {}
pub fn get_current_dir(&self) -> Option<&path::Path> {}

This is my first rust PR, so apologies if anything is against convention.

@epage I'm not sure which additional getters you meant by

plus ones specialized for the extra content in assert_cmd::Command.

as unless I'm missing something, there aren't any setters that aren't also in std

• Most of this is liberally copied from std, but I noticed many of the existing doc blocks were also either the same or had significant overlap. I'm happy to trim them down or just write new ones
• I haven't added any new unit tests as I couldn't see any similar ones / the rest of the suite seems to be more integration test style
• I left in the doctests and didn't add no_run like many of the other examples (and have verified that they pass correctly)
• Similarly to std, I grouped the getters at the end. I can change them to be colocated if preferred

[coveralls]

Pull Request Test Coverage Report for Build 10309787777

Details

• 0 of 8 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 44.581%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/cmd.rs	0	8	0.0%

Totals
Change from base Build 10190508249:	-0.7%
Covered Lines:	218
Relevant Lines:	489

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💛 - Coveralls

[epage]

This is good to go once clippy is resolved. Feel free to squash the fix.