Support previous and next links with non-standard content

Added option to use classes on the list items to identify previous
and next links, to give more flexibility and allow use of custom
content (eg icons, text, etc) in the link element content.
Add composer.json to allow simple integration in php projects

This will make it easier for end-users to manage the dependency alongside
other vendor libraries and packages in a PHP project.
Merge branch 'feat/configurable-next-or-prev'