fix(package.json): list optional dependencies

This will enable `npm shrinkwrap` to work on projects that (transitively)
depend on chokidar.

On Linux a harmless warning is displayed during `npm install` this is fine:

```
npm WARN optional dep failed, continuing fsevents@0.2.0
```
Merge pull request #106 from IgorMinar/optional-deps

fix(package.json): list optional dependencies