Add views_prefix Option For Alternative Views Directory

# Summary

This PR adds a new option for `#paginate` and friends to specify an alternative views directory. While the default points to `app/views/kaminari`, users can now specify their own.

```ruby
paginate @users, views_prefix: "../pagination"
```

Partials are now searched in `app/pagination/`. This is especially useful in the Cells gem.

# Implementation

I extracted partial path logic into `Tag#partial_path`.

During my work I found some more places where we could remove redundant code (e.g. as found in `Tag#initialize` and its successor `Paginator#initialize`. I'll submit a separate PR for that :heart: