Support custom auto-activation files. Fixes #5.

This changes the default auto-activation filename from .vfenv to the
more standard .venv, while at the same time providing a setting that
can be used to customize this filename to any arbitrary value. For any
folk who already have .vfenv files, this setting can be used to provide
backwards compatibility.
Merge pull request #7 from justinmayer/auto-activation-file

Support custom auto-activation files. Fixes #5.