Move `horizontal`, `vertical` in separate helpers
Reduce public API methods from HorizontalHelper

Before this PR, including `bh` in an app would include more methods
than necessary for horizontal: methods like `horizontal_string`
that should only be accessed privately.
Reduce public API methods from VerticalHelper

Before this PR, including `bh` in an app would include more methods
than necessary for vertical: methods like `vertical_string` and
`toggle_button` that should only be accessed privately.

This PR extracts those private methods into a new Vertical class
that includes all the business logic to render elements and edit
attributes (e.g., attach classes), leaving the helper cleaner.
Merge pull request #64 from claudiob/extract-horizontal-and-vertical-helpers

Extract horizontal and vertical helpers