Add xonsh shell support

[anki-code]

It will be cool to have an ability to set the shell (e.g. https://xon.sh) i.e.:

xonsh
$SHELL_GENIE_SHELL="xonsh"
shell-genie ask "set env variable HELLO with "world" value"  # automatically added "on xonsh"
# Command: $HELLO="world"
# Do you want to run this command? [y/n]:

[b0bd3v]

I think this can be modified in the config file.

[dylanjcastillo]

Yes, @b0bd3v is correct.