make Virtualenvs optional

[Saphyel]

I think this should be a minor change and it would be a big improvemnt for people using containers or people that doesn't know how to use Virtualenvs

[mitsuhiko]

I hope that it’s possible to make virtualenvs as obvious and clear as node_modules or the rust target folder. By adding a second non virtualenv mode the entire user experience will become more complex as now a user needs to understand both modes.

[dgqyushen]

totally agree