Add pyvenv-activate as a safe local variable and use it when loading dir-locals to activate a specific virtualenv.

I *think* this fixes #12.

It does use the pyvenv-activate variable first as it more explicit. For
example, I expected the code to do something like this:

  (defun pyvenv-workon (name)
    (pyvenv-activate (find-venv-path-in-virtualenvs name)))

With that in mind, specifying the pyvenv-activate just skips the look
up, hence it takes precedence.