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.
It seems like it should be either or. I don't think you should need to build first, then start watching.