Release Newsboat 2.16

Lists below only mention user-visible changes, but I would also like to
acknowledge contributions from Ivan Tham.

Added
=====
- Install changelog and contrib/ alongside docs (Alexander Batischev) (#474)
- `show-title-bar` config option to hide the title bar. Defaults to yes, i.e.
    the behaviour is the same as with Newsboat 2.15 (Sermak) (#375)
- Contrib scripts for image preview (Sermak) (#480)
- Nord colour scheme (Daryl Manning)
- Ability to search withing the search results, narrowing them down (Tumlinh)
    (#327)
- Color scheme based on Adapta-Maia GTK theme (Lucas Parsy)

Changed
=======
- Marking feed as read only resets the cursor if article list is sorted by date
    (Stefan Assmann)
- `include` also accepts relative paths (Marco Sirabella) (#489)
- Update vendored version of nlohmann/json to 3.6.1
- Update vendored version of Catch2 to 2.9.1

Fixed
=====
- Parser breaking on spaces inside backticks (Marco Sirabella) (#492)
- Hidden tags changing the title of their feeds (Alexander Batischev) (#498)
- Segfaults some time after using an invalid regex in a filter expression
    (Alexander Batischev) (#501)
- Single quotes in podcast names replaced by %27 (屑鉄さらい;Scrap Trawler)
    (#290, #457)
- Out-of-bounds access on empty "author" tag in RSS 0.9x (Alexander Batischev)
    (#542)
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