add console to supported globals, for easy logging

This PR makes it possible to do this sort of thing:

```js
ractive = new Ractive({
  el: 'body',
  template: '{{ console.log("the answer is " + (a+b) ) }}',
  data: { a: 40, b: 2 }
});
```

Makes it nice and easy to debug stuff. Can anyone think of a good reason *not* to do this?