Release Newsboat 2.17

Lists below only mention user-visible changes, but I would also like to
acknowledge contributions from the following people: Raphael Nestler, kpcyrd,
and seanBE.

Added
=====
- FreeBSD and Linux i686 jobs on continuous integration servers. We won't break
    the build on these platforms ever again (Alexander Batischev)
- Documentation for `macro-prefix` settings (Lysander Trischler)
- `save-all` operation, to save all articles in the feed (Romeu Vieira)
- `dirbrowser-title-format` setting, used in the DirBrowser dialog invoked by
    `save-all` operation (Romeu Vieira)
- `dirbrowser` context in `bind-key` command, to add bindings to DirBrowser
    dialog invoked by `save-all` (Alexander Batischev)
- `selecttag-format` setting, to control how the lines in "Select tag" dialog
    look (Penguin-Guru, Alexander Batischev) (#588)

Changed
=======
- Bumped minimum required Rust version to 1.26.0
- Update vendored version of nlohmann/json to 3.7.0
- Update vendored version of Catch2 to 2.9.2
- Update Italian translations (Leandro Noferini)

Removed
=======
- Some unnecessary work done at startup time, shaving off 6% in my tests
    (Alexander Batischev)

Fixed
=====
- `newsboat --version` not displaying the version (Alexander Batischev) (#579)
- Processing backticks inside comments (Jan Staněk)
- Use-after-free crash when opening an article (Juho Pohjala) (#189)
- Crash on `toggle-item-read` in an empty feed (Nikos Tsipinakis)
- Un-applying a filter when command is ran (Nikos Tsipinakis) (#607)
- Numerous memory leaks detected by Clang's AddressSanitizer (Alexander
    Batischev) (#620, #621, #623, #624)
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