Edit pyenv virtualenv-init check to support non-brew installations
Edit pyenv virtualenv-init check to support non-brew installations

If the pyenv virtualenv plugin is installed using the [pyenv-installer][1]
app script or directly via a [Git clone][2], then the pyenv-virtualenv-init
executable, that the `pyenv virtualenv-init` command uses, will not
exist in the user's PATH and therefore cannot be found using
`$commands[pyenv-virtualenv-init]`.
Installing the pyenv-virtualenv plugin in this manner is common among Linux users.

Using the pyenv `commands` command, which lists all commands pyenv can
run, we can find if the virtualenv-init command is available to pyenv
and subsequently the virtualenv plugin, without relying on
pyenv-virtualenv-init to exist in the user's PATH.

[1]: https://github.com/pyenv/pyenv-installer
[2]: https://github.com/pyenv/pyenv-virtualenv#installing-as-a-pyenv-plugin

ysfjwd/prezto