Hello,

After a clean install today, I took great care to set up my dev environment and this time around I made sure that pip wouldn't run at all if I'm not in a virtualenv. I believe this is one of the lesser known features of pip and it can be extremely useful to forgetful people like me who start working on something in a virtualenv and they forget to activate that same virtualenv again when they come back to it some time later.

If this seems like a reasonable addition, I can write it up and create a PR but would love some feedback first.

Thanks!