Improve inline virtualenv activation logic

[uranusjr]

Set VIRTUAL_ENV during activation so py.exe does the right thing.

Before:

> pipenv run py -c "import sys; print(sys.executable)"
C:\Users\uranusjr\AppData\Local\Programs\Python\Python36\python.exe

After:

> pipenv run py -c "import sys; print(sys.executable)"
C:\Users\uranusjr\Documents\programming\pipenv\.venv\Scripts\python.exe

Also add venv support—it is actually quite easy, so why not.

[uranusjr]

I’m not sure why VIRTUAL_ENV was not set before. Maybe there is a reason I am not aware of?