Python virtualenv auto workon cwd hook (#1300)

* Command to activate virtualenvs on changing directory

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.

runcoms/zpreztorc

@@ -107,6 +107,13 @@ zstyle ':prezto:module:prompt' theme 'sorin'
 # Auto switch the Ruby version on directory change.
 # zstyle ':prezto:module:ruby:chruby' auto-switch 'yes'
 
+#
+# Python
+#
+
+# Auto switch the Python virtualenv on directory change.
+# zstyle ':prezto:module:python:virtualenv' auto-switch 'yes'
+
 #
 # Screen
 #