--list option with simple functional test

With optional prefix arg, because nothing else sensibly comes after
--list anyway, and the list might be very long.

I don't verify that anything 'really is' a virtualenv beyond isdir,
that is a slippery slope fraught with edge cases, and IMO people shouldn't
put other junk dirs in WORKON_HOME/ve_base anyway
Merge pull request #33 from sashahart/list_option

This seems pretty uncomplicated in practice