add a --version option to the hook loader

[dhellmann]

Use the hook loader --version option to report the version of
virtualenvwrapper installed when the user runs the virtualenvwrapper
command.

Fixes #42

[dhellmann]

cc @nedbat

[dhellmann]

On my system, the new output looks like

virtualenvwrapper

virtualenvwrapper is a set of extensions to Ian Bicking's virtualenv
tool.  The extensions include wrappers for creating and deleting
virtual environments and otherwise managing your development workflow,
making it easier to work on more than one project at a time without
introducing conflicts in their dependencies.

For more information please refer to the documentation:

    http://virtualenvwrapper.readthedocs.org/en/latest/command_ref.html

Version: 6.0.0.0a2.dev1
Script: /Users/dhellmann/Devel/virtualenvwrapper/virtualenvwrapper/virtualenvwrapper.sh
Python: /Library/Frameworks/Python.framework/Versions/3.10/bin/python3.10
WORKON_HOME: /Users/dhellmann/Envs
PROJECT_HOME: /Users/dhellmann/Devel

Commands available:
...