Can't pass `class` to `link_to` helper

The built-in [`link_to`](https://hexo.io/docs/helpers.html#link-to) helper allows a `class` to be passed into it, but it doesn't appear the Handlebars version allows that to happen. I would like to be able to do something like this:

``` handlebars
{{link_to page.next "Next" class="next"}}
```

Start with the Handlebars implementation of the link_to helper and compare its supported arguments with the built-in link_to documentation linked in the issue. Confirm that the shown class="next" usage renders a link with the requested class, and add or update coverage if the repository has tests for this helper.

Written by the indexing model from the issue text.