Add scrollToHash setting

Usually, scrollNav will try to scroll to `window.location.hash`. But with hash-based navigation, which is used by Backbone.js and other Javascript frameworks, it will lead to errors, because it will try to scroll to, for example, `#about/contribute`, which is absolutely wrong element selector.
This pull request keeps original behavior, but allows to disable scrolling to an element which ID is equal to `window.location.hash`