Changes:
* Simplify zstyle name `skip-virtualenvwrapper-init` to `initialize`
avoiding double negation in name
* Always perform `eval (pyenv virtualenv-init -)` at initialization
* Prefer `virtualenvwrapper_lazy` over `virtualenvwrapper` when available
* Honor `VIRTUALENVWRAPPER_VIRTUALENV` if it is defined.
* Document about `VIRTUALENVWRAPPER_PYTHON` and `VIRTUALENVWRAPPER_VIRTUALENV`
(this would be particularly important in macOS after recent homebrew update)
* Add additional documentation for `initialize` in _README.md_ and _zpreztorc_
* Add aliases `py2`, `py3` as shortcut for `python2`, `python3` respectively
Indrajit Raychaudhuri