Improve enclosure support
Fulfills enhnacement request fguillot/miniflux #155
Inserts a (unread/total)" items count in front of each subcription
on the feeds page.

models/feed.php adds two functions: get_feed_unread_counts(), get_all_w_counts().
get_feed_unread_counts - returns either an array of (unread/total) counts indexed
by feed id or a singl (unread/total) for a specific id.
get_all_w_counts() returns the same as get_all() but appends items_unread and
items_total columns.

controller/feed.php modifies Router\post_action('refresh-feed',function() to
retrun the (unread/total) count in the retruned ajax json array.
Router\get_action('feeds',function() was modified to call get_all_w_counts().

templates/feeds.php was modified to insert the (unread/total) count into the
span id="loading-feed-" such that is can be updated after a refresh.

assets/js/feed.js was modified to to have the Update and hideRefreshIcon
update the returned (unread/total) amounts in the "loading-feed" span.