added ability to write plugins directly in moonboots config

Once I needed an ability to write custom plugins for stylus through the moonboots. For example, I wanted to define global variable with path to images dir. But with existing functionality of stylizer I can't do like this:

```javascript
stylizer({
          infile: cssDir + '/app.styl',
          outfile: cssDir + '/app.css',
          development: true,
          watch: cssDir + '/**/*.styl',
          plugins: [
            function(stl) {
              stl.define('$imgpath', config.client.prefix + config.client.public + 'images/');
            }
          ]
        }, done);
```

I added few lines of code to provide the ability to write plugins in this way.