Manage SnapURL paths sensibly

Grabbing the link, headline and trail text, and having to use `grunt-prompt`, every time is a bit of a pain and is open to mistakes. This PR makes those variables part of `source.json`. It generates something a little like this...
```json
{
  "html": "<div class=\"test__wrapper\"><style></style><h1>If you can read this then something has gone wrong</h1></div>",
  "previous": "",
  "refreshStatus": true,
  "url": "http://www.theguardian.com",
  "headline": "The Guardian",
  "trailText": "Latest news, sport and comment from the Guardian"
}
```

This means that you only need to enter these values once when running `grunt new`. If you wish to update these values at a future date, run `grunt update`.

PATHS!

![200](https://cloud.githubusercontent.com/assets/1607666/5185347/ae15658e-74b6-11e4-82f5-1d90a27dd3f4.gif)