Improve virtualenv extension: handle vim-pyenv

[vim-pyenv](https://github.com/lambdalisue/vim-pyenv) handles pyenv's
virtualenvs / Pythons.

/cc @lambdalisue
> VIRTUAL_ENV is the final fallback if other plugins are not available

Yes, but it limits the condition, and my use case has been to show vim-pyenv's information always, which does not require an activated virtualenv - but then that does not fit this extension.
I'll revert this - also the `airline_enable_virtualenv` change, which I've seen in other statements around.