capture_output is not on by default in the standard library - replicate this behavior

[klardotsh]

This can be seen even in a repl - stdlib subprocess.run('clang') will pass through the error about no input files, this library will swallow it because it sets stderr/stdout to subprocess.PIPE by default. This PR fixes that.

Same drill as my other two PRs, not 100% clean. Includes #4. Can rebase.

[klardotsh]

Will fix tests some day that's not today - interesting that this suddenly fails when the result I'm seeing on screen is what one would expect.

[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.