Add support for creating the venv using pyvenv if it is available in the active Python environment and the user explicitly set a variable to indicate they prefer pyvenv when possible.
README.md update about PYENV_VIRTUALENVWRAPPER_PREFER_PYVENV
Merge pull request #16 from eukaryote/feature/support_pyvenv

Feature: support pyvenv
Add tests for `pyvenv` (#16)