add testenv-configuration for installer command
pip and easy_install both support --find-links as an alternative to using an index server.
we use --find-link stores at my company to store our local distributions.
as it is, to use tox, we have to add the findlink directives to the pip config for the user account running tox.
instead of just adding options to pip/easy_install one by one you could consider just offering an "advanced" option to allow users to construct their own pip/easy_install commands in the config.