Add validation for command options

This PR fixes [issue #2299](https://github.com/stefanpenner/ember-cli/issues/2299). The command model now parses the command options and will throw a warning if the option does not exist in the knownOptions hash. For example:

```
ember serve --envirnment=production
version: 0.1.2
The option '--envirnment' is not supported by the serve command. Run `ember serve --help` for a list of supported options.
Livereload server on port 35729
Serving on http://0.0.0.0:4200/
```

I'm working on tests and will update the branch with them shortly. I mainly just wanted to get the PR up for review.