Add suffix and prefix options to virtualenv plugin

Following the prefix/suffix pattern used by [`git_prompt_info()`](https://github.com/robbyrussell/oh-my-zsh/blob/master/lib/git.zsh#L5)

You can set:
```
ZSH_THEME_VIRTUALENV_PROMPT_PREFIX
ZSH_THEME_VIRTUALENV_PROMPT_SUFFIX
```
in a theme and these will wrap the virtualenv prompt information

If these aren't set, the plugin behaves exactly as before.

Also, added virtualenv plugin support to the ["blinks"](https://github.com/robbyrussell/oh-my-zsh/blob/master/themes/blinks.zsh-theme) theme
theme behaves normally if plugin is not activated

/cc @bigjust