Add python and pip environment variables

[anki-code]

Hi! To make xonsh.AppImage more independent I suggest:

1. Set environment variables which make python and pip works from AppImage:

# it will be great if '~/.pip-appimage' would be '~/.pip-appimage-name' for example '~/.pip-appimage-xonsh'
export PIP_TARGET='~/.pip-appimage'
export PYTHONPATH=$PIP_TARGET

PATH=$PYTHONHOME/bin:$PATH

2. If it is possible add to sys.path:

sys.path.append('~/.pip-appimage')

After this python and pip will work from xonsh.AppImage by default without any additional changes.