Early support for event callbacks

Allows user to have event handlers in the nodemon config, such as:

```json
{
  "events": {
    "crash": "say 'your app crashed'",
    "start": "echo 'App started' | tee app.log"
  }
}
```

https://github.com/remy/nodemon/wiki/Events#states

Fixes #432.

## Todo

- [ ] CLI support :-\
- [ ] Check parsing of commands
- [ ] Check windows