Made it possible to use the plugin like `.use(markdown)`

In the [README](https://github.com/segmentio/metalsmith) of metalsmith you use the markdown plugin without bracket like this:

```js
 Metalsmith(__dirname)
    .use(markdown);
    .build([...])
```

But this isn't implemented right now. 

This PR will use `.apply` to delegate the function call correctly. 
 
Basically it is just doing this:
```js
if ( arguments.length === 3){
  plugin().apply(null, [].slice.call(arguments));
}
```