prevent disclosing full path when raising "Shaarli directory not writeable" error
 * work on https://github.com/shaarli/Shaarli/issues/78
prevent disclosing PHP version on PHP version check error
 * fixes https://github.com/shaarli/Shaarli/issues/78
 * fixes https://github.com/sebsauvage/Shaarli/issues/214
Merge pull request #81 from nodiscc/prevent-disclosure

Prevent full path and PHP version disclosures
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