update rainTPL to 2.7.2
plugin system: add LINKLIST_PLUGINS global array to config.
 * this is a list of all plugins that will be loaded when rainTPL renders the link list items
plugin system: let rainTPL parse and render enabled plugins
 * each plugin is a raintpl file in tpl/plugins/$pluginname.html
plugin system: add example plugins
 * readityourself is an actual plugin, requested at https://github.com/sebsauvage/Shaarli/issues/66
 * the other 3 plugins are dummy test plugins