Example code fails silently due to plugins

Same as https://github.com/segmentio/metalsmith.io/pull/5

If plugins are required and passed in to `#use` without having their constructor called, the metalsmith `#build` method fails silently.

eg:

```js
var Metalsmith = require('metalsmith')
  , markdown = require('metalsmith-markdown');

Metalsmith(__dirname)
  .use(markdown)
  .build(function(err) {
    console.log('This will never be printed');
  });
```

Is it potentially worth storing a hash of plugin methods registered to be called and then error on `build` if they haven't all been called, signifying one in the `use` chain failed to call `next`?