Expose `Element.Input.Label` type

[smt116]

Allow to use Label type outside of the style-elements package. As the result, it is possible to create helper functions that uses Label type for their adnotations.

For example:

placeholderConfig : String -> Input.Label style variation msg
placeholderConfig placeholderText =
    Input.placeholder
        { label = Input.hiddenLabel ""
        , text = placeholderText
        }

[cjduncana]

What about adding documentation to the Label type?

[mdgriffith]

Thanks!