Enables automatic activation of a virtualenv when jumping into a directory
This is done by looking in the current directory for a file or directory
named `.venv`. If it's not found in the current directory its parents will
also be examined. `.venv` can be either:
1. A file containing the name of a virtualenv found in $WORKON_HOME
2. A directory containing bin/activate (meaning that the directory is
assumed to be a virtualenv.)
If $WORKON_HOME is set it is assumed that virtualenvwrapper is installed
and the `workon` command will be issued.
