Function to "link" package.json scripts to launch tasks

There should be something like `launch --link` that updates the local `package.json`.

Writes something like this to `package.json`:

``` js
"scrips": {
"build": "launch build",
"test": "launch test"
}
```

For all launch tasks that match:
- `build`, `start`, `test`
- `pre*`, `post*`