Updated patch following Gregory's suggestions:

- The check_returncode parameter is now called check. The method on CompletedProcess is still check_returncode, though.
- Clarified the docs about args
- CalledProcessError and TimeoutExceeded gain a stdout property as an alias of output

Ethan: to combine stdout and stderr in check_output, you need to pass stderr=subprocess.STDOUT - it doesn't assume you want that.

I did consider having a simplified interface so you could pass e.g. capture='combine', or capture='stdout', but I don't think the brevity is worth the loss of flexibility.