Plugin system

This adds a working plugin system, that loads templates based on the  ̀LINKLIST_PLUGINS` config variable. Example:

`$GLOBALS['config']['LINKLIST_PLUGINS'] = array('readityourself', 'archiveorg');` loads extra templates from `tpl/plugins/readityourself/readityourself.html` and `tpl/plugins/archiveorg/archiveorg.html`

For now it is only possible to add plugins to the linklist area where date/permalink/qrcode are.

archive.org integration added recently has been ported to a plugin. This also adds a "readityourself" plugin requested in https://github.com/sebsauvage/Shaarli/issues/66.

I also updated rainTPL to 2.7.2 from https://raw.githubusercontent.com/rainphp/raintpl/master/inc/rain.tpl.class.php.