Allow plugins to be used in stylus.render options

Allows a function or array of functions to be passed as the `use` option to `stylus.render`.

```js
var plugin = function(key, value) {
  return function(style) {
    style.define(key, new stylus.nodes.Literal(value));
  }
};

stylus.render('body { foo: bar  }', {
  use: plugin('bar', 'baz')
});

// body { foo: baz }
```