Changes to link URL

[rmzelle]

As agreed on at #23 (comment):

• Use a link icon (e.g. https://octicons.github.com/icon/link/, which GitHub uses when hovering over anchored headers, e.g. https://github.com/citation-style-language/styles/blob/master/CONTRIBUTING.md#guide-to-submitting-csl-styles-and-csl-locales) instead of the text "Link"?
• Show target URL when hovering over a "Link" link (like any regular link does in Chrome), probably by adding a "href" attribute to them? That would also make it possible to go use the browser's back button to deselect a style.