Reduce public API methods from LinkToHelper

Before this PR, including `bh` in an app would include more methods
than necessary for link_to: methods like `add_link_class!` and
`nav_item_options` that should only be accessed privately.

This PR extracts those private methods into a new LinkTo class
that includes all the business logic to render elements and edit
attributes (e.g., attach classes), leaving the helper cleaner.