Allow to declare inside tox.ini that some environments need to not use a venv

As this plugin scope is to bypass the usage of a venv with tox, I do have a highly related use-case: I want to add some tox environments in `tox.ini` file that will not use any venv.

Basically this is asking for another way to activate this plugin instead of the command line switch.

This could prove very useful for using tox as a more generic build tool.