Add new API method: `getFeedStats`

This patch is intended to support an Emacs Gnus backend for ttrss that I'm writing.
I don't know your policy on accomodating consumers of your API; this change does not modify existing code, but merely adds two functions. Thanks for considering it. Here are some details:

`getFeedStats` calls the new static function `api_get_feed_stats` to
return an array of real feeds of the following form:

    [{
       "first" : 1127,
       "unread" : 873,
       "last" : 15460,
       "title" : "Some feed",
       "id" : 31,
       "total" : 1513
    }]

where "first", "last", "total" are the first, last and total number of
articles in the feed.

This adds the ability to nntp-oriented clients to efficiently retrieve
an "active file" without having to resort to `getHeadlines(limit=-1)`