[enh] add offline engine for sqlite database

To test & demonstrate this implementation download:

  https://liste.mediathekview.de/filmliste-v2.db.bz2

and unpack into searx/data/filmliste-v2.db, in your settings.yml define a sqlite
engine named "demo"::

    - name : demo
      engine : sqlite
      shortcut: demo
      categories: general
      result_template: default.html
      database : searx/data/filmliste-v2.db
      query_str :  >-
        SELECT title || ' (' || time(duration, 'unixepoch') || ')' AS title,
               COALESCE( NULLIF(url_video_hd,''), NULLIF(url_video_sd,''), url_video) AS url,
               description AS content
          FROM film
         WHERE title LIKE :wildcard OR description LIKE :wildcard
         ORDER BY duration DESC
      disabled : False

Query to test: "!demo concert"

This is a rewrite of the implementation from commit [1]

[1] searx/searx@8e90a21

Suggested-by: @virtadpt searx/searx#2808

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