Add prefix/suffix options to virtualenv prompt

Following the pattern used in `git_prompt_info()`:
https://github.com/robbyrussell/oh-my-zsh/blob/master/lib/git.zsh#L5
Allow `virtualenv_prompt_info()` to (optionally) accept a prefix and
suffix set in the theme
Add virtualenv plugin support to 'blinks' theme

If the plugin is not activated, degrade gracefully