Shorter display of a single virtualenv item

[amw]

At some point type labels like py[ENV] were added to the virtualenv display. I find this to be very verbose in most situations where people only use a single virtual environment in a given directory. I've modified my local copy to remove type label, please compare before and after:

I would suggest either:

1. Dropping the prefix entirely
2. Dropping prefixes where there is only one virtualenvs item
3. Using separate segments for different virtualenvs

I can submit PR for the first or second option.