Provide virtualenv support with a behavior similar to current git / rvm suport

New Functionnality:
====

Files
----

  + `plugins/virtualenv/virtualenv.plugin.zsh`: 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`

Configuration
---

  + `ZSH_THEME_VIRTUALENV_PROMPT_PREFIX`
  + `ZSH_THEME_VIRTUALENV_PROMPT_SUFFIX`
These variables are used to customize the output of `virtualenv_prompt_info`



Sample Usage
===

    ZSH_THEME_VIRTUALENV_PROMPT_PREFIX="[%{$fg_bold[blue]%}venv:%{$fg_bold[green]%}"
    ZSH_THEME_VIRTUALENV_PROMPT_SUFFIX="%{$reset_color%}] "

When on a Virtualenv called "MyEnv": `[venv:MyEnv]`  (+ colors!)


Credits
====

This plugin is similar to and inspired by [this pull request](https://github.com/robbyrussell/oh-my-zsh/pull/1223) but offers a higher level of customization and strives to be in line with current similar functionnality.