New Functionnality:
+ plugins/virtualenv/... plugin: stop virtualenv from prepending
'(virtualenv_name)' to the prompt
+ lib/virtualenv.zsh: provide a virtualenv_prompt_info function that
behaves similarly to lib/git.zsh's git_prompt_info
Uses:
+ ZSH_THEME_VIRTUALENV_PROMPT_PREFIX
+ ZSH_THEME_VIRTUALENV_PROMPT_SUFFIX
These variables are added to the output of virtualenv_prompt_info
