Feature: support pyvenv

This pull request includes a tiny change to  ```bin/pyenv-virtualenvwrapper``` to support getting virtualenvwrapper's ```mkvirtualenv``` command to create the environment using the stdlib's ```pyvenv``` if the Python environment has a ```bin/pyvenv``` and the user explicitly sets a new variable ```PYENV_VIRTUALENVWRAPPER_PREFER_PYVENV``` to indicate that they prefer pyvenv when it's available. The variable by default will be absent, so behavior will be unchanged for users unless they explicitly set the variable.

I also added a brief description of the option to the README.md.