Add virtualenv plugin

[chawyehsu]

A tiny plugin to display virtualenv in prompt.

Usage example: https://github.com/h404bi/dotfiles/blob/master/src/pshazz/h404bi.json

{
    "plugins": [ "virtualenv" ],
    "prompt": [
        [ "darkgreen",   "", "$hostname" ],
        [ "darkcyan",    "", " $dir" ],
        [ "darkcyan",  "", " $virtualenv_lbracket$virtualenv"],
        [ "darkcyan",  "", "$virtualenv_rbracket"],
        [ "",            "", "`n$" ]
    ],
    "virtualenv": {
        "prompt_lbracket": "[",
        "prompt_rbracket": "]"
    }
}

What it looks like:

And please make a new release of pshazz for scoop if this pull request being merged.

[lukesampson]

Cool, thank you.