Capture subprocess stdout by default

[joouha]

This PR sets the standard output of subprocess to be captured by default in the xonsh kernel.

This means that the output of external commands will sent to connected kernel clients, instead of being displayed in the terminal where the kernel is running.

[anki-code]

hi @joouha!
This is not working for me:

xonsh
xpip install -U git+https://github.com/joouha/xontrib-jupyter@fix-subproc-stdout
cd /tmp
euporie-notebook --kernel-name xonsh

If I run this many times:

• sometimes I see just 1
• sometimes I see just 1 2
• sometimes I see just 1 <ls result> (without 2)

[anki-code]

@joouha what do you think, why we have unstable output?