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.
I'm assuming that the change handler fires on a file `change`, so it wouldn't detect changes since the last time `build` was run.

Eg:- `ember build` run 10 minutes ago, followed by `ember build --watch`. In this case the `onchange`  event won't fire since no file has changed since the time the `--watch` command started.

Hence, `build` then `watch`.