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
add wallabag sharing plugin
 * uses wallabag's ?plainurl action to store in wallabag
 * configuration of base wallabag URL is done through the WALLABAG_URL global variable
 * fixes https://github.com/sebsauvage/Shaarli/issues/167
start moving qrcode to plugin system
 * remove qr specific code for linklist template
 * add html item to qrcode.linklist.html
 * add the script itself to qrcode.footer.html
add "playvideos" plugin: adds a "Play videos" toolbar button
     * code from https://github.com/zaius/youtube_playlist/
     * Plays all Youtube videos linked from the page in an overlay HTML5 player
     * See it's README.md
use a single PLUGINS list to enable plugins
 * tpl/plugins/*/*.toolbar.html will add elements to the toolbar
 * tpl/plugins/*/*.linklist.html will add elements to the linklist
plugins: add empty linklist/toolbar templates to prevent raintpl from failing
 * each plugin should have .toolbar.html and .linklist.html else RainTPL fails at finding the file and stops parsing
plugins: use the PLUGINS variable in tpl/linklist.html instead of obsolete LINKLIST_PLUGINS
plugins: move qr code as a plugin
 * add plugin parsing loop in footer (required to include the qrcode script)
 * move javascript in it's own qrcode.js file (work on #33) and load it from qrcode.footer.html
 * enable qr plugin by default
 * move qr icon in plugin directory
 * add empty qrcode.toolbar.html to prevent raintpl parsing failure
plugins: change qr plugin name to qrcode
Improvements to the plugin system
 * Rendering the linklist has been moved to a separate function (to allow greater customisation if the need arises)
 * A PageBuilder is now created every time we render the page. Although it is not always page settings, like we'll do with plugins. Also, this removes the potential errors when forgetting to create a pageBuilder and so on.
 * We now build a list of plugins for each location (for now: toolbar, linklist, footer) so that RTPL has no issues rendering them
 * Simplified plugins templates. Linklist templates can now access the link info through the  variable, while toolbar and footer can access global settings via .
 * This means we don't have to create empty files anymore to prevent raintpl from crashing