Automatically retrieve description for new bookmarks

If the option is enabled, it will try to find a meta tag containing
the page description and keywords, just like we do for the page title.
It will either look for regular meta tag or OpenGraph ones.

The option is disabled by default.

Note that keywords meta tags is mostly not used.

In `configure` template, the variable associated with this setting
is `$retrieve_description`.

Fixes #1302

doc/md/Shaarli-configuration.md

@@ -56,6 +56,8 @@ _These settings should not be edited_
 - **timezone**: See [the list of supported timezones](http://php.net/manual/en/timezones.php).  
 - **enabled_plugins**: List of enabled plugins.
 - **default_note_title**: Default title of a new note.
+- **retrieve_description** (boolean): If set to true, for every new links Shaarli will try
+to retrieve the description and keywords from the HTML meta tags.
 
 ### Security