Add xonsh command syntax to work with xonsh as the default shell

[eugenesvk]

I've added a new Python command syntax that can be understood by the Python-based xonsh shell, which is required when xonsh is used as the default shell since it chokes on the current sh-based syntax:

• via a -p/--python command line argument (but then this breaks other default shells, so inconvenient if you switch default shells)
• via an auto-detection feature (which runs a grep "^$(id -un)": /etc/passwd to get the default shell before launching the relevant command) — I've copied your main process creation code and just removed the parts that weren't needed (sockets etc.), so it's a bit of a code duplication

For the new syntax I bypassed the appendWslArg function as there is no need to escape the quotes, and I don't understand how relevant the other corrections are (and how to adjust them properly for the new syntax) :)

Only tested this with ConEmu and msys2

Fixes: #31

[Biswa96]

Thank you for submitting your pull request. I appreciate your contribution and the time you have invested in making this change. However, I am not currently interested in implementing this change. I apologize for any inconvenience this may cause.