Xontrib-argcomplete

* Added debug mode: `$XONTRIB_ARGCOMPLETE_DEBUG=True`
* Fixed completion in the line beginning

Package maintenance

Added shebang support:

./script shebang should has "env python" or "env xonsh"
script script should be found in $PATH and shebang should has "python" or "xonsh"

Added xonsh support:

./script.xsh <Tab>
./path/script.xsh <Tab>
xonsh script.xsh <Tab>

Added workaround for [xonsh/issues/3394](https://github.com/xonsh/xonsh/issues/3394)

Added support for Python version.

Example:
bash
python3.6 script.py <Tab>