[Feature Request] Theme API: add has_tag helper to coreHelpers. 

According to your observations made on #1525

* Added has_tag helper to coreHelpers foundation.
* Added unit tests to check has_tag works as expected.

This patch adds functionality to support the following use case:

 ```html
        <section class="post-excerpt">
          {{#has_tag "snippet"}}
                <p class="code-snippet">{{content}}</p>
          {{ else }}
            <p>{{excerpt}}&hellip;</p>
          {{/has_tag}}
        </section>
```

This could be pretty useful to filter post types using tags.