Allow multiple `end` event handlers + optional result css modification

For example:

`autoprefixer.js`
```javascript
var autoprefixer = require('autoprefixer');

var plugin = function() {
    return function(style) {
        this.on('end', function(err, css) {
            return autoprefixer.compile(css);
        });
    };
};
module.exports = plugin;
```

`csso.js`
```javascript
var csso = require('csso');

var plugin = function() {
    return function(style) {
        this.on('end', function(err, css) {
            return csso.justDoIt(css);
        });
    };
};
module.exports = plugin;
```

`style.styl`
```css
use('autoprefixer.js')
use('csso.js')

body
  box-shadow: 3px 3px 5px #ccc
  transform: scale(2)
```

`style.css` (result)
```css
body{-webkit-box-shadow:3px 3px 5px #ccc;box-shadow:3px 3px 5px #ccc;-webkit-transform:scale(2);-ms-transform:scale(2);transform:scale(2)}
```