Adds --watch flag to `ember build`

This PR adds the `--watch` flag to the `build` command. The implementation is more or less the same. I've moved around the `run` method into the `build` method. The `watcher` calls `this.build` in the change handler.

Usage is,

```bash
ember serve --watch
```
The `watch` flag is false by default.
Just noticed this, had used skip. Is it possible to recover from this, or should I send a new PR?