Add HBS template helper so handlebars partials are not evaluated as a Hugo Template

I have multiple projects that leverage handlebars partials being rendered in in layouts as templates:

```
        <template id="footer">
          {{ hbs "globals/footer.hbs" }}
        </template>
```

This template helper will allow partials to be rendered as handlebars without trying to evaluate and interpolate the variables in the template language.

A pull would be great, I'm also open to changing this to `rawPartial` or adding `rawPartial` as an alias for the helper.