Accept nonpresent links

Currently, if the page is loaded with a hashlink for which the target element is present on the page but there is no link to it present, skrollr-menu will prevent the browser from scrolling to the element as it should, but not replicate the functionality. 
This commit will create a link to the correct element and then activate it if there is not already a link present. 
I would prefer to only create a single in-memory link and reuse it. So basically move the `createElement` to the initialization and only update the `href` here