Automatically retrieve description for new bookmarks

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