Add .use method for chaining plugins.

Trivial change but makes it feel more like you're performing a
sanctioned activity rather than hacking an exercise object.

Converts this:

```js
exercise = execute(exercise)
exercise = comparestdout(exercise)
exercise = boilerplate(exercise)
exercise = wrappedexec(exercise)
```

To this:

```js
exercise = exercise.use(execute)
                   .use(comparestdout)
                   .use(boilerplate)
                   .use(wrappedexec)
```