Add a basic plugin system
 * 2 new global config options: LINKLIST_PLUGINS and TOOLBAR_PLUGINS
 * These options are lists of plugins that will be loaded when rainTPL renders the link list and toolbar, respectively
 * each plugin is a raintpl template in tpl/plugins/$pluginname/$pluginname.html
  * if the plugin contains multiple files, they must be self contained in it's directory
  * this allows unpacking plugins form a zip or cloning them from git
 * add readityourself plugin, requested at https://github.com/sebsauvage/Shaarli/issues/66
  * This plugin adds, for each link, a button to display a more readable version using an external readityourself service
 * move archive.org integration as a plugin. To enable, add 'archiveorg' to the LINKLIST_PLUGINS array
 * styling fixes for plugins
 * remove archive.org plugin styling from main CSS, use icon instead
 * match archiveorg/readityourself plugins styling with qrcode
 * Icon from http://p.yusukekamiyamane.com/icons/search/fugue/icons/clock-history-frame.png under CC-BY-SA License
 * Icon from http://p.yusukekamiyamane.com/icons/search/fugue/icons/book-open.png under CC-BY-SA License