Provide returncode and check_returncode

[klardotsh]

Just some more backports. Like #7, this also includes #4 from @brbsix. Open to a clean rebase, but the library is broken for my purposes without it so for now I left the commit in there.

https://docs.python.org/3/library/subprocess.html#subprocess.CompletedProcess.returncode (this is basically what status was already doing here)
https://docs.python.org/3/library/subprocess.html#subprocess.CompletedProcess.check_returncode

[klardotsh]

Closing this to clear out my Pull Requests view on GitHub. Looks like Textio's fork is still public, though I haven't worked there in years so can't speak to its maintenance story, whether it's still in use, etc. I'll leave any future upstreaming efforts to them if they so desire.