Fixes issue #186: test case tagging

You can now tag a test case by adding the `tags` property to a test module.

```
module.exports = {
  tags: ['login', 'sanity'],
  'demo login test': function (client) {
    // test code
  }
};
```

Select tags to run by specifying the `--tag` flag (`-a` for short):

```
bin/nightwatch --tag login --tag something_else
```